[jfx21u] RFR: 8343136: Change JavaFX release version to 21.0.6 in jfx21u

[Johan Vos]

Bump the release version in .jcheck/conf and build.properties to 21.0.6

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Commit messages:
 - 8343136: increase release version to 21.0.6

Changes: https://git.openjdk.org/jfx21u/pull/73/files
  Webrev: https://webrevs.openjdk.org/?repo=jfx21u&pr=73&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8343136
  Stats: 2 lines in 2 files changed: 0 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jfx21u/pull/73.diff
  Fetch: git fetch https://git.openjdk.org/jfx21u.git pull/73/head:pull/73

PR: https://git.openjdk.org/jfx21u/pull/73