golang: enable 1.21

[v1v]

What

Enable 1.21 in main since it was released in early August 2023 (https://go.dev/blog/go1.21)

Task

• Branch off 1.20 from main
• Use 1.21.0 golang version
• Replace 1.19 to 1.20

Issues

Similar to #223

Further details

See changelog for 1.21.0

@elastic/ingest-fp this is required when a new minor version is released as explained in https://github.com/elastic/golang-crossbuild#releasing-images-for-a-new-go-version. Can you please review this?

[mergify]

This pull request does not have a backport label. Could you fix it @v1v? 🙏
To fixup this pull request, you need to add the backport labels for the needed
branches, such as:

• backport-v./d./d is the label to automatically backport to the 1./d branch. /d is the digit
  NOTE: backport-skip has been added to this pull request.

[elasticmachine]

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• Start Time: 2023-08-14T18:39:26.449+0000

• Duration: 123 min 34 sec

🤖 GitHub comments

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To re-run your PR in the CI, just comment with:

• /test : Re-trigger the build.

[v1v]

As far as I see gvm in the CI runners is not using the latest version so it cannot pick up https://github.com/andrewkroh/gvm/releases/tag/v0.5.1

[v1v]

The important golang versions are defined in the dockerfiles.

This version is only required for running the go template.go in the host.

env.GO_VERSION is used for knowing when a new release needs to happen

[amannocci]

LGTM