Revert junit-jupiter v6 bump in JDK8-pinned modules

[jbachorik]

Summary

• PR fix(deps): update dependency org.junit.jupiter:junit-jupiter to v6 #871 (Renovate) rewrote three explicit resolutionStrategy.force pins on org.junit.jupiter:junit-jupiter (btrace-agent, btrace-gradle-plugin, integration-tests) from JDK8-compatible versions (5.14.4 / 5.11.4) to v6.1.2.
• Those pins exist specifically because these modules' test sourcesets target JDK8, and junit-jupiter 6.x requires JDK17+ to compile against — the comments right above each pin explain this. Renovate isn't aware of that constraint and blindly bumped the pinned version anyway.
• This has broken develop's build job since the merge (Could not resolve org.junit.jupiter:junit-jupiter:6.1.1 ... only compatible with JVM runtime version 17 or newer), and it merged in the first place because develop has no required status checks configured, so Renovate's automerge never actually waited for CI.

Fix

Straight revert of #871 — restores the three pins to their JDK8-compatible versions. Verified :btrace-agent:compileTestJava and :integration-tests:compileTestJava both succeed locally.

Test plan

• ./gradlew :btrace-agent:compileTestJava :integration-tests:compileTestJava — BUILD SUCCESSFUL
• CI green on this PR

Separately, I'm adding required status checks to develop's branch protection so this class of break can't merge again regardless of Renovate's automerge settings.

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This change is