RFR: 8325093: Update CONTRIBUTING.md for build jdk version

[Ao Qi]

[JDK-8297068](https://bugs.openjdk.org/browse/JDK-8297068) updated boot JDK to 19.0.1 from 18.0.2, and [JDK-8321434](https://bugs.openjdk.org/browse/JDK-8321434) updated boot JDK to 21.0.1. `CONTRIBUTING.md` should be updated correspondingly.

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Commit messages:
 - 8325093: Update CONTRIBUTING.md for build jdk version

Changes: https://git.openjdk.org/jfx/pull/1357/files
 Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=1357&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8325093
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jfx/pull/1357.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/1357/head:pull/1357

PR: https://git.openjdk.org/jfx/pull/1357