RFR: 7861: Bump Agent build to Java 11 compliance and cleanup POM [v3]

[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 three additional commits since the last revision:

 - Delete .gitignored org.eclipse.jdt.core.prefs
 - Merge branch 'master' into tryouts-agent
 - Cleanup JMC agent pom

-------------

Changes:
  - all: https://git.openjdk.org/jmc/pull/415/files
  - new: https://git.openjdk.org/jmc/pull/415/files/b144c770..9ba93841

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jmc&pr=415&range=02
 - incr: https://webrevs.openjdk.org/?repo=jmc&pr=415&range=01-02

  Stats: 843 lines in 204 files changed: 99 ins; 382 del; 362 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