Add Spring Boot 4 support

[cretz]

BEFORE MERGE:

• Merge Add opt-in Jackson 3 support via multi-release JAR #2783
• Modify this PR to target the main branch (currently set to jackson-3 branch for easier review).

What was changed

• Updated Gradle build files and added CI test to ensure Spring 4 works
  • Did not have to change code
• Updated MetricsScopeAutoConfiguration and OpenTracingAutoConfiguration to point to the newer Spring 4 class names
  • Also added test to that would have broken on Spring 4 without these changes

Leans on #2783 (since both are in same fork, hard to target it without forcing PR to be on the fork too, will update when other merged, for now ignore the Jackson 3 parts).

Checklist

1. Closes Spring Boot 4 Support #2738
2. Closes Spring Metrics Configuration fails with Spring Boot 4 #2758

[mjameswh]

[unrelated] Seems like removing this line is causing test flakes in Nexus tests, not sure why.

[mjameswh]

Unrelated, but this was causing Gradle error messages in my environment because jgit used internally by com.palantir.git-version doesn't know how to deal with Git worktrees. AFAICS, we're anyway relying on CLI calls to git, rather than use APIs provided by that plugin.