Bump go to v1.21.6 and set a single source of truth

[rikatz]

What this PR does / why we need it:

This PR:

• Sets a single source of truth for go version (the GOLANG_VERSION file)
• Bump go version to v1.21.6
• Uses this go version already during controller compilation, without needing to bump test-runner

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• CVE Report (Scanner found CVE and adding report)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation only

Which issue/s this PR fixes

How Has This Been Tested?

Checklist:

• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I've read the CONTRIBUTION guide
• I have added unit and/or e2e tests to cover my changes.
• All new and existing tests passed.

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[k8s-ci-robot]

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If Ingress contributors determines this is a relevant issue, they will accept it by applying the triage/accepted label and provide further guidance.

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[rikatz]

/cherry-pick release-1.9

[k8s-infra-cherrypick-robot]

@rikatz: once the present PR merges, I will cherry-pick it on top of release-1.9 in a new PR and assign it to you.

In response to this:

/cherry-pick release-1.9

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the kubernetes/test-infra repository.

[tao12345666333]

/lgtm

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: rikatz, tao12345666333

The full list of commands accepted by this bot can be found here.

The pull request process is described here

Needs approval from an approver in each of these files:

• OWNERS [rikatz,tao12345666333]

Approvers can indicate their approval by writing /approve in a comment
Approvers can cancel approval by writing /approve cancel in a comment

[k8s-infra-cherrypick-robot]

@rikatz: #10926 failed to apply on top of branch "release-1.9":

Applying: Bump go to v1.21.6 and set a single source of truth
.git/rebase-apply/patch:39: trailing whitespace.
        
.git/rebase-apply/patch:62: trailing whitespace.
      
.git/rebase-apply/patch:96: trailing whitespace.
      
.git/rebase-apply/patch:211: trailing whitespace.
  exit 1 
warning: 4 lines add whitespace errors.
Using index info to reconstruct a base tree...
M	.github/workflows/ci.yaml
M	.github/workflows/golangci-lint.yml
A	.github/workflows/images.yaml
M	.github/workflows/plugin.yaml
M	Makefile
M	build/run-in-docker.sh
M	go.mod
M	go.work.sum
A	images/Makefile
M	images/custom-error-pages/rootfs/Dockerfile
M	images/ext-auth-example-authsvc/rootfs/Dockerfile
M	images/fastcgi-helloserver/rootfs/Dockerfile
M	images/go-grpc-greeter-server/rootfs/Dockerfile
M	images/httpbun/rootfs/Dockerfile
M	images/kube-webhook-certgen/rootfs/Dockerfile
M	images/test-runner/Makefile
M	magefiles/go.mod
Falling back to patching base and 3-way merge...
Auto-merging magefiles/go.mod
Auto-merging images/test-runner/Makefile
Auto-merging images/kube-webhook-certgen/rootfs/Dockerfile
Auto-merging images/kube-webhook-certgen/Makefile
CONFLICT (content): Merge conflict in images/kube-webhook-certgen/Makefile
Auto-merging images/httpbun/rootfs/Dockerfile
Auto-merging images/go-grpc-greeter-server/rootfs/Dockerfile
Auto-merging images/fastcgi-helloserver/rootfs/Dockerfile
Auto-merging images/ext-auth-example-authsvc/rootfs/Dockerfile
Auto-merging images/custom-error-pages/rootfs/Dockerfile
Auto-merging go.work.sum
CONFLICT (content): Merge conflict in go.work.sum
Auto-merging go.mod
Auto-merging build/run-in-docker.sh
Auto-merging Makefile
Auto-merging .github/workflows/plugin.yaml
CONFLICT (modify/delete): .github/workflows/images.yaml deleted in HEAD and modified in Bump go to v1.21.6 and set a single source of truth. Version Bump go to v1.21.6 and set a single source of truth of .github/workflows/images.yaml left in tree.
Auto-merging .github/workflows/golangci-lint.yml
Auto-merging .github/workflows/ci.yaml
CONFLICT (content): Merge conflict in .github/workflows/ci.yaml
error: Failed to merge in the changes.
hint: Use 'git am --show-current-patch=diff' to see the failed patch
Patch failed at 0001 Bump go to v1.21.6 and set a single source of truth
When you have resolved this problem, run "git am --continue".
If you prefer to skip this patch, run "git am --skip" instead.
To restore the original branch and stop patching, run "git am --abort".

In response to this:

/cherry-pick release-1.9

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the kubernetes/test-infra repository.