Investigate renovate not updating Dockerfile/Makefile go versions

[chancez]

@lambdanis had to manually update the Go version in the Dockerfile and Makefile in the v0.12 stable branch here #1317 because Renovate didn't.

I started looking into it, and I verified renovate can parse the comments, but for some reason, it isn't seeing any updates available. If you remove the allowedVersions constraint, it's showing two updates for Go, one for the latest 1.20 release and another for the latest 1.21 release. I tried various allowedVersions such as <1.21 <=1.20, etc but simply by setting allowedVersions it stopped showing updates.

I tested by checking out the v0.12 branch, commenting out the matchBaseBranchVersions for the golang stable group, manually adjusting the Makefile/Dockerfile's Go version to something older (meaning it needs to be updated) and running:

docker run --rm -ti -e LOG_LEVEL=debug -e GITHUB_COM_TOKEN="$(gh auth token)" -v /tmp:/tmp -v ${PWD}:/usr/src/app docker.io/renovate/renovate:full renovate --platform=local | tee renovate.log

[lambdanis]

For reference, in #1310 Renovate updated Go to 1.21.5 on main branch, and this PR seems to be complete. #1314 updated Go to 1.20.12 on v1.12 branch, but only in CI workflows.