RFR: 7526: JMC not buildable with jdk17 [v3]

[Johannes Bechberger]

> Tested on Java 11 x86 and aarch64 (azul, but only builds), and on Java 17 x86 and aarch64.

Johannes Bechberger 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 four additional commits since the last revision:

 - Merge branch 'openjdk:master' into parttimenerd_java17
 - Fix maven cache directories in GitHub actions
 - Update READMEs and GitHub actions to include JDK 17
 - Update mockito and p2-maven-plugin to work with Java 17

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

Changes:
  - all: https://git.openjdk.java.net/jmc/pull/363/files
  - new: https://git.openjdk.java.net/jmc/pull/363/files/1018eae1..f43d9506

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jmc&pr=363&range=02
 - incr: https://webrevs.openjdk.java.net/?repo=jmc&pr=363&range=01-02

  Stats: 547 lines in 45 files changed: 289 ins; 102 del; 156 mod
  Patch: https://git.openjdk.java.net/jmc/pull/363.diff
  Fetch: git fetch https://git.openjdk.java.net/jmc pull/363/head:pull/363

PR: https://git.openjdk.java.net/jmc/pull/363