RFR: 8269597: Change JavaFX release version to 18

[Kevin Rushforth]

Bump the version number of JavaFX to 18. I will integrate this immediately after forking the `jfx17` stabilization branch, which is scheduled for Thursday, July 8, 2021 at 16:00 UTC.

Leaving it as `Draft` for now. I'll make it `rfr` next week.

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Commit messages:
 - 8269597: Change JavaFX release version to 18

Changes: https://git.openjdk.java.net/jfx/pull/554/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jfx&pr=554&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8269597
  Stats: 4 lines in 2 files changed: 0 ins; 0 del; 4 mod
  Patch: https://git.openjdk.java.net/jfx/pull/554.diff
  Fetch: git fetch https://git.openjdk.java.net/jfx pull/554/head:pull/554

PR: https://git.openjdk.java.net/jfx/pull/554