RFR: 8345136: Update JDK_DOCS property to point to JDK 23 docs

[Kevin Rushforth]

Update the docs bundle to point to the JDK 23 docs (we're currently pointing to JDK 21).

I will need to also download and host a zip bundle and add its checksum to gradle/verification-metadata.xml to avoid breaking our internal CI build.

As mentioned in [JDK-8325873](https://bugs.openjdk.org/browse/JDK-8325873) when we updated the docs to use JDK 21, we should do this as a part of updating the boot JDK. To facilitate this, and to move more version number properties to a common place, I will create a new `jdk.docs.version` property in `build.properties` and put it near the `jfx.build.jdk.version` property.

Both the GHA build and our  CI build are successful.

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Commit messages:
 - Move loading of build.properties earlier in build.gradle
 - 8345136: Update JDK_DOCS property to point to JDK 23 docs

Changes: https://git.openjdk.org/jfx/pull/1663/files
  Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=1663&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8345136
  Stats: 13 lines in 3 files changed: 7 ins; 2 del; 4 mod
  Patch: https://git.openjdk.org/jfx/pull/1663.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/1663/head:pull/1663

PR: https://git.openjdk.org/jfx/pull/1663