x/build/cmd/coordinator: warn if a slowbot is requested on a Go version that is known to be unsupported by the builder

[dmitshur]

If we can determine that the following two conditions are true:

• a builder requires at least Go 1.N or at most Go 1.M (e.g., via the MinimumGoVersion field or similar)
• the commit being asked for testing represents a Go version that is outside the builder's requirement (e.g., by using information in internal/goversion package or similar)

Then coordinator can print a warning about it when that builder is explicitly requested as a slowbot.

See #41829 for motivation.

/cc @ianlancetaylor @golang/release @cagedmantis @toothrot