Update golang Docker tag to v1.21.5 (main)

[renovate]

This PR contains the following updates:

Package	Type	Update	Change
golang	final	patch	1.21.4-bullseye -> 1.21.5-bullseye

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[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:pr6868-e4be452cf9`. This can take up to 1 hour.

[charleskorn]

LGTM pending build image update

[github-actions]

Not building new version of mimir-build-image. This PR modifies mimir-build-image/Dockerfile, but the image grafana/mimir-build-image:pr6868-e4be452cf9 already exists.

[renovate]

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You can manually request rebase by checking the rebase/retry box above.

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[aknuds1]

@charleskorn there's some sort of bug in the GitHub workflow, so after the new image tag is committed, one has to rebase the PR and push or some such, to re-trigger the CI checks.