[jfx22] RFR: 8325873: Update JDK_DOCS property to point to JDK 21 docs

[Kevin Rushforth]

Clean backport of a docs-only fix to jfx22.

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Commit messages:
 - Backport f379eae65aab963779d7b7335cbade810f8efe82

Changes: https://git.openjdk.org/jfx/pull/1369/files
 Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=1369&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8325873
  Stats: 4 lines in 2 files changed: 0 ins; 0 del; 4 mod
  Patch: https://git.openjdk.org/jfx/pull/1369.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/1369/head:pull/1369

PR: https://git.openjdk.org/jfx/pull/1369