RFR: 2112: Update JDK to 21 and Gradle to 8.4

[Erik Joelsson]

Bumping JDK version to 21.0.1 and Gradle version to 8.4.

There were two tests that needed to be updated. One was assuming iteration order on HashMap. The other was more complex as it was triggered by [JDK-8289689](https://bugs.openjdk.org/browse/JDK-8289689). My conclusion is that we can't count on unicode normalization returned from Git to match what the files in the filesystem have on mac, and so the test needs to accommodate for that. There is a longer explanation in a comment.

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

Commit messages:
 - new gha
 - Simplified unicode test
 - SKARA-2112

Changes: https://git.openjdk.org/skara/pull/1590/files
 Webrev: https://webrevs.openjdk.org/?repo=skara&pr=1590&range=00
  Issue: https://bugs.openjdk.org/browse/SKARA-2112
  Stats: 35 lines in 4 files changed: 15 ins; 0 del; 20 mod
  Patch: https://git.openjdk.org/skara/pull/1590.diff
  Fetch: git fetch https://git.openjdk.org/skara.git pull/1590/head:pull/1590

PR: https://git.openjdk.org/skara/pull/1590