[jfx23u] Integrated: 8304008: Update README.md and CONTRIBUTING.md for jfx update repos

[Kevin Rushforth]

This is a copy of the jfx22u README and CONTRIBUTING docs, changing 22 to 23, with one additional change to add the word "Updates" after "23" in one place where it was missing (the current wording could mislead developers into thinking that JavaFX 23 fixes should be done here).

I'll take this out of Draft once PR #1 is integrated (that PR _must_ be integrated ahead of anything else and before I announce that the repo is open for pull requests).

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Commit messages:
 - Clarify that PRs are for 23 'Updates' (not 23)
 - Update README.md and CONTRIBUTING.md for jfx update repos

Changes: https://git.openjdk.org/jfx23u/pull/2/files
  Webrev: https://webrevs.openjdk.org/?repo=jfx23u&pr=2&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8304008
  Stats: 269 lines in 2 files changed: 1 ins; 256 del; 12 mod
  Patch: https://git.openjdk.org/jfx23u/pull/2.diff
  Fetch: git fetch https://git.openjdk.org/jfx23u.git pull/2/head:pull/2

PR: https://git.openjdk.org/jfx23u/pull/2