RFR: 8283517: Update boot JDK to 18

[Kevin Rushforth]

JDK 18 has been released, so we should update the default boot JDK used to build JavaFX to JDK 18. This will not affect the minimum boot JDK version. I have confirmed that gradle 7.3 works with JDK 18, so we don't need to update gradle at this time.

I have done a full set of tests on all three platforms.

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Commit messages:
 - 8283517: Update boot JDK to 18

Changes: https://git.openjdk.java.net/jfx/pull/760/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jfx&pr=760&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8283517
  Stats: 11 lines in 2 files changed: 0 ins; 0 del; 11 mod
  Patch: https://git.openjdk.java.net/jfx/pull/760.diff
  Fetch: git fetch https://git.openjdk.java.net/jfx pull/760/head:pull/760

PR: https://git.openjdk.java.net/jfx/pull/760