RFR: 8277475: Update JDK_DOCS property to point to JDK 17 docs

[Kevin Rushforth]

We are still pointing to the JDK 12 API docs when building the JavaFX docs. This PR updates the build to point to the JDK 17 API docs.

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Commit messages:
 - 8277475: Update JDK_DOCS property to point to JDK 17 docs

Changes: https://git.openjdk.java.net/jfx/pull/686/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jfx&pr=686&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8277475
  Stats: 4 lines in 2 files changed: 0 ins; 0 del; 4 mod
  Patch: https://git.openjdk.java.net/jfx/pull/686.diff
  Fetch: git fetch https://git.openjdk.java.net/jfx pull/686/head:pull/686

PR: https://git.openjdk.java.net/jfx/pull/686