feat(pd): update go version to 1.21

[purelind]

pd master update go version to 1.21.

Ref tikv/pd#6923

[ti-chi-bot]

I have already done a preliminary review for you, and I hope to help you do a better job.
Pull Request Review

Summary:
This pull request aims to update the Go version used in the PD Master pipeline from 1.20 to 1.21. The change is implemented by updating the Golang container image used in the pipeline.

Potential problems:

• There are no potential problems with this change.

Fixing suggestions:

• None needed.

Overall, this is a simple and straightforward change that updates the Go version used in the pipeline. Therefore, I suggest approving this pull request.

[purelind]

Ref tikv/pd#6924

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

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