Use Go 1.21

[adriansmares]

Summary

This PR upgrades our stack to be built using Go 1.21.

Changes

• Use Go 1.21 in go.mod and CI.
• Use standard slices package instead of experimental one.
• Use standard maps package instead of experimental one, mostly.
  • Keys and Values have been removed last moment from the 1.21 release, and will be added back in a future version.

Testing

CI. No explicit testing is required.

Regressions

Based on my reading of the 1.21 changelog, there shouldn't be any breaking changes, and our experimental package usage was already aligned more or less with the 1.21 interface.

Checklist

• Scope: The referenced issue is addressed, there are no unrelated changes.
• Compatibility: The changes are backwards compatible with existing API, storage, configuration and CLI, according to the compatibility commitments in README.md for the chosen target branch.
• Documentation: Relevant documentation is added or updated.
• The steps/process to test this feature are clearly explained including testing for regressions.
• Changelog: Significant features, behavior changes, deprecations and fixes are added to CHANGELOG.md.
• Commits: Commit messages follow guidelines in CONTRIBUTING.md, there are no fixup commits left.