[CI:BUILD] Add golang 1.21 update warning

[cevich]

This is needed on the off-chance that some tool or a human suggests updating the minimum version to 1.21 or later. Since doing so would cause Fedora and Debian to start behaving differently WRT builds.

Ref: containers/automation#187

Does this PR introduce a user-facing change?

None

[packit-as-a-service]

Cockpit tests failed for commit 4b295e0ab49ffe649cda4f6a5ffe79863a6669fc. @martinpitt, @jelly, @mvollmer please check.

[cevich]

@nalind @rhatdan @mtrmac @Luap99 PTAL. If this language is acceptable, I'd like to dupe this change into Buildah and Skopeo as well as any other golang repos you think important.

See also containers/automation#187

[packit-as-a-service]

Cockpit tests failed for commit f5a4b1fa9182ab5f188cb4df9226cd95d214e71e. @martinpitt, @jelly, @mvollmer please check.

[packit-as-a-service]

Cockpit tests failed for commit d10cbe89fbbc3ba0e6e6380f089bc5a4b5d8ae2d. @martinpitt, @jelly, @mvollmer please check.

[mtrmac]

I do like the idea of the warning.

It might be a bit clearer to explicitly separate the two:

• go is set to the latest go of all dependencies. That can’t be prevented, other than not updating the dependency.
• toolchain is bumped when increasing go for some reason (and based on the currently running tools, not on what any dependency declares). If, at the time of the go bump (or later), we choose a toolchain version acceptable to us, it is not going to be auto-updated until a go auto-update happens.

[cevich]

@nalind @rhatdan @mtrmac @Luap99 PTAL. I've updated the comment text based on Miloslav's comments/corrections. Assuming it's correct now, after this merges, I intend to make a similar change in buildah and skopeo (and any other Renovate-managed repos you think are important).

[mtrmac]

This LGTM

Meanwhile, https://bodhi.fedoraproject.org/updates/?packages=golang suggests that F38 now does support Go 1.21. So maybe we can update soon — but that is not a reason to not merge this PR right now.

[mtrmac]

/lgtm

[ashley-cui]

/approve

[openshift-ci]

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