RFR: 7526: JMC not buildable with jdk17 [v4]

[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 incrementally with two additional commits since the last revision:

 - Improve devguide
 - Update jetty versions

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

Changes:
  - all: https://git.openjdk.java.net/jmc/pull/363/files
  - new: https://git.openjdk.java.net/jmc/pull/363/files/f43d9506..639c4575

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jmc&pr=363&range=03
 - incr: https://webrevs.openjdk.java.net/?repo=jmc&pr=363&range=02-03

  Stats: 10 lines in 2 files changed: 4 ins; 0 del; 6 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