RFR: 8255: Update toolchain and base images to support building with Java21 in docker

[Martin Skarsaune]

Getting this trying to build with docker from master:


[ERROR] Failed to execute goal org.apache.maven.plugins:maven-toolchains-plugin:3.2.0:toolchain (default) on project missioncontrol: Cannot find matching toolchain definitions for the following toolchain types:
[ERROR] jdk [ id='JavaSE-21' version='21' ]
[ERROR] Please make sure you define the required toolchains in your ~/.m2/toolchains.xml file.
[ERROR] -> [Help 1]

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Commit messages:
 - Also simplify p2 a bit
 - Update toolchain and base images to support building with Java21 in docker

Changes: https://git.openjdk.org/jmc/pull/585/files
  Webrev: https://webrevs.openjdk.org/?repo=jmc&pr=585&range=00
  Issue: https://bugs.openjdk.org/browse/JMC-8255
  Stats: 21 lines in 3 files changed: 14 ins; 2 del; 5 mod
  Patch: https://git.openjdk.org/jmc/pull/585.diff
  Fetch: git fetch https://git.openjdk.org/jmc.git pull/585/head:pull/585

PR: https://git.openjdk.org/jmc/pull/585