java2x: migrate killbill-plugin-api to jakarta baseline

[Copilot]

This aligns killbill-plugin-api with the Java 21 / Jakarta migration baseline used by killbill-commons:0.27.0. In this repo, no source-level javax.* → jakarta.* or Jooby artifact migration was required; the remaining work was build-baseline alignment at the root POM.

• Baseline alignment (root POM)

  • Set Java target baseline to 21:
    • project.build.targetJdk=21
  • Added commons alignment property:
    • killbill-commons.version=0.27.0

• Java 21 Enforcer compatibility

  • Added centralized property:
    • extra-enforcer-rules.version=1.12.0
  • Wired maven-enforcer-plugin to use that helper dependency at the root level (no module-level version drift).

• Namespace / Jooby audit outcome

  • Repository-wide audit found no targeted framework-facing javax.* usages to migrate in source.
  • No org.jooby:* dependencies/modules were present, so no killbill-jooby substitution was needed in this repository.

<properties>
  <extra-enforcer-rules.version>1.12.0</extra-enforcer-rules.version>
  <killbill-commons.version>0.27.0</killbill-commons.version>
  <project.build.targetJdk>21</project.build.targetJdk>
</properties>

[xsalefter]

Yeah this one doesn't have any jakarta namespace.