chore: Bump Golang to 1.21.8

[jannfis]

Bump Golang in Dockerfiles to 1.21.8.

We cannot use 1.22 yet, see kubernetes-sigs/kubebuilder#3792 - Our codegen will fail NPEs.

Checklist:

• Either (a) I've created an enhancement proposal and discussed it with the community, (b) this is a bug fix, or (c) this does not need to be in the release notes.
• The title of the PR states what changed and the related issues number (used for the release note).
• The title of the PR conforms to the Toolchain Guide
• I've included "Closes [ISSUE #]" or "Fixes [ISSUE #]" in the description to automatically close the associated issue.
• I've updated both the CLI and UI to expose my feature, or I plan to submit a second PR with them.
• Does this PR require documentation updates?
• I've updated documentation as required by this PR.
• I have signed off all my commits as required by DCO
• I have written unit and/or e2e tests for my change. PRs without these are unlikely to be merged.
• My build is green (troubleshooting builds).
• My new feature complies with the feature status guidelines.
• I have added a brief description of why this PR is necessary and/or what this PR solves.
• Optional. My organization is added to USERS.md.
• Optional. For bug fixes, I've indicated what older releases this fix should be cherry-picked into (this may or may not happen depending on risk/complexity).

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 49.72%. Comparing base (05eea87) to head (94e7cd5).
Report is 51 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #17616      +/-   ##
==========================================
+ Coverage   49.44%   49.72%   +0.27%     
==========================================
  Files         273      274       +1     
  Lines       48662    48939     +277     
==========================================
+ Hits        24063    24334     +271     
+ Misses      22237    22230       -7     
- Partials     2362     2375      +13     

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[ishitasequeira]

LGTM