RFR: 7526: JMC not buildable with jdk17 [v15]

[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 pull request now contains 13 commits:

 - Merge branch 'master' into parttimenerd_java17
 - Remove exports workaround
 - Fix formatting
 - Improve GitHub actions
 - Fix PropertyContentBuilderTest
   
   Related to differences in events between JDK11 and later JDKs
 - Mention JVMKeepAlive
   
   Using a predefinied launcher does not work properly (in my setting)
 - Improve devguide
 - Bump javadoc version
 - Add another -version
 - Add JDK 17 to CI
 - ... and 3 more: https://git.openjdk.java.net/jmc/compare/95fe7394...dc5af324

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

Changes: https://git.openjdk.java.net/jmc/pull/363/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jmc&pr=363&range=14
  Stats: 95 lines in 5 files changed: 47 ins; 24 del; 24 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