renovate-bot.md

This allows projects to opt out again from the truncated versions by
overriding the configuration to use full-semver again.

The annoying thing with this implementation is that we need to keep
the list in `renovate-truncated-versions.json` and
`renovate-no-truncated-versions.json` in sync. It might be better to
sprinkle this with some jsonnet, but both resulting files would need
to be checked in anyway for consumption by renovate downstream.

Follow up to https://gitlab.com/gitlab-com/gl-infra/common-ci-tasks/-/merge_requests/909#note_2355440070