[1.x] Deprecate useJCenter key in 1.x

### Discussed in https://github.com/sbt/sbt/discussions/7761

<div type='discussions-op-text'>

<sup>Originally posted by **Philippus** October 13, 2024</sup>
I think the JCenter resolver (jcenter.bintray.com) now redirects to Maven. Should it be removed/deprecated?</div>

See also https://github.com/sbt/sbt/pull/7801