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

[Kevin Rushforth]

On Wed, 14 Feb 2024 15:19:47 GMT, Kevin Rushforth <kcr at openjdk.org> wrote:

> 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.

Reviewer: @arapte

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PR Comment: https://git.openjdk.org/jfx/pull/1366#issuecomment-1944048236