Log an actionable warning/error when build is configured to CI publish snapshots to Sonatype Central

[armanbilge]

Spinning out of https://github.com/typelevel/sbt-typelevel/pull/739#discussion_r1746518728.

When all of the following are true:

1. tlUntaggedAreSnapshots == true
2. sonatypeCredentialHost == "central.sonatype.com"
3. tlCiReleaseBranches.size > 0

The build is "invalid" because it is not possible to publish -SNAPSHOTs to the Sonatype Central Host and thus the CI publish job will fail.

https://central.sonatype.org/faq/snapshot-releases/

We should catch this early and log a warning or error the build for a better UX. It just wasn't clear to me where exactly to check for this 🤔