Build container using x.y.z golang version and updating Golang version

[razo7]

Why we need this PR

Simplify how the Dockerfile fetches the Golang version for building the binary.

Changes made

• Fetch the Golang version from the go directive in go.mod, and download it using the repos of the base image (CentOS stream 8) instead of using the official website to download and extract the newer Golang version
• Upgrade Golang 1.20-> 1.21.7 and use the patch version in go.mod
• Modify ci-operator.yaml file for Golang builder in OCP CI.

Which issue(s) this PR fixes

Follow up to #132, and should not be merged beforehand.

[openshift-ci]

Skipping CI for Draft Pull Request.
If you want CI signal for your change, please convert it to an actual PR.
You can still manually trigger a test run with /test all

[openshift-ci]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: razo7

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[slintes]

/test 4.15-test

[razo7]

/test 4.15-test

[razo7]

/test 4.15-test

[razo7]

/test 4.15-test

[razo7]

/test 4.15-openshift-e2e

[razo7]

There is a tradeoff for using a patch version (here and eventually aligning it in other places) for Golang builder, and the pros won't overcome the cons. It might be more relevant in the future, but not for now.