RFR: 7526: JMC not buildable with jdk17

[Johannes Bechberger]

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

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Commit messages:
 - Update READMEs and GitHub actions to include JDK 17
 - Update mockito and p2-maven-plugin to work with Java 17

Changes: https://git.openjdk.java.net/jmc/pull/363/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jmc&pr=363&range=00
  Issue: https://bugs.openjdk.java.net/browse/JMC-7526
  Stats: 90 lines in 5 files changed: 80 ins; 0 del; 10 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