RFR: 8325873: Update JDK_DOCS property to point to JDK 21 docs

[Kevin Rushforth]

Simple fix to update the pointer to the JDK docs to JDK 21 (matching the current boot JDK). The only impact of this change is that hyperlinks in the generated API docs will link to the online JDK 21 docs. As this is a docs-only change, I intend to backport this to the jfx22 stabilization branch during RDP2.

-------------

Commit messages:
 - 8325873: Update JDK_DOCS property to point to JDK 21 docs

Changes: https://git.openjdk.org/jfx/pull/1366/files
 Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=1366&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/1366.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/1366/head:pull/1366

PR: https://git.openjdk.org/jfx/pull/1366