chore(deps): update golang docker tag to v1.21.8 (main)

[pr00se]

What this PR does

Update mimir-build-image to go 1.21.8 outside of renovate-bot so we can get it into the next release.

Which issue(s) this PR fixes or relates to

N/A

Checklist

• Tests updated.
• Documentation added.
• CHANGELOG.md updated - the order of entries should be [CHANGE], [FEATURE], [ENHANCEMENT], [BUGFIX].
• about-versioning.md updated with experimental features.

[github-actions]

Building new version of mimir-build-image. After image is built and pushed to Docker Hub, a new commit will automatically be added to this PR with new image version grafana/mimir-build-image:pr7557-ae15c572b6. This can take up to 1 hour.

[github-actions]

Not building new version of mimir-build-image. This PR modifies mimir-build-image/Dockerfile, but the image grafana/mimir-build-image:pr7557-ae15c572b6 already exists.

[grafanabot]

The backport to r280 failed:

The process '/usr/bin/git' failed with exit code 1

To backport manually, run these commands in your terminal:

# Fetch latest updates from GitHub
git fetch
# Create a new branch
git switch --create backport-7557-to-r280 origin/r280
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick -x e0a86e6c54e6467b27c638e1655fec5a7fc4d658
# Push it to GitHub
git push --set-upstream origin backport-7557-to-r280
git switch main
# Remove the local backport branch
git branch -D backport-7557-to-r280

Then, create a pull request where the base branch is r280 and the compare/head branch is backport-7557-to-r280.

[grafanabot]

The backport to r280 failed:

The process '/usr/bin/git' failed with exit code 1

To backport manually, run these commands in your terminal:

# Fetch latest updates from GitHub
git fetch
# Create a new branch
git switch --create backport-7557-to-r280 origin/r280
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick -x e0a86e6c54e6467b27c638e1655fec5a7fc4d658
# Push it to GitHub
git push --set-upstream origin backport-7557-to-r280
git switch main
# Remove the local backport branch
git branch -D backport-7557-to-r280

Then, create a pull request where the base branch is r280 and the compare/head branch is backport-7557-to-r280.