RFR: 7861: Bump Agent build to Java 11 compliance and cleanup POM [v2]

[Christoph Langer]

> The JMC Agent project is set up in a somewhat awkward way at the moment. The maven.compiler.source and maven.compiler.target properties are set to 1.8 but we enforce a JDK 11 during the build and also have a test case that implicitly uses/tests JDK11 features. Maybe it is time now to bump the agent to Java 11 compliance?
> 
> Furthermore, the POM file could be cleaned up and a few maven plugins that are used could be bumped to more recent versions.

Christoph Langer has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains one additional commit since the last revision:

  Cleanup JMC agent pom

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Changes:
  - all: https://git.openjdk.org/jmc/pull/415/files
  - new: https://git.openjdk.org/jmc/pull/415/files/6e16c710..b144c770

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jmc&pr=415&range=01
 - incr: https://webrevs.openjdk.org/?repo=jmc&pr=415&range=00-01

  Stats: 10446 lines in 206 files changed: 9020 ins; 770 del; 656 mod
  Patch: https://git.openjdk.org/jmc/pull/415.diff
  Fetch: git fetch https://git.openjdk.org/jmc pull/415/head:pull/415

PR: https://git.openjdk.org/jmc/pull/415