RFR: 2112: Update JDK to 21 and Gradle to 8.4 [v2]

[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.

Erik Joelsson has updated the pull request incrementally with one additional commit since the last revision:

  Updated readme and build.gradle

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Changes:
  - all: https://git.openjdk.org/skara/pull/1590/files
  - new: https://git.openjdk.org/skara/pull/1590/files/67a9b615..3a0c04a6

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=skara&pr=1590&range=01
 - incr: https://webrevs.openjdk.org/?repo=skara&pr=1590&range=00-01

  Stats: 21 lines in 2 files changed: 4 ins; 1 del; 16 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