Use latest Java toolchain and simplify Javadoc generation now that the javadoc tool can properly link to older docs

[pantherdd]

Use latest Java toolchain and simplify Javadoc generation now that the javadoc tool can properly link to older docs with the following tickets fully complete:

• JDK-8216497 – javadoc should auto-link to platform classes
• JDK-8297437 – javadoc cannot link to old docs (with old style anchors)

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The only remaining issue is Gradle's own Javadocs, in that

• it would currently require HTML4 format fragments at the end of the links (e.g. Project.html#getName--),
• but it tricks the new, smart javadoc implementation into generating them in HTML5 format (e.g. Project.html#getName()),
• so the deep links don't work properly.

I will try to ask the Gradle maintainers to fix this before I merge this PR. The possible solutions:

• They could also start using Java 17+'s javadoc and generate modern HTML5, which would also fix their links to JDK classes
  • https://github.com/gradle/gradle/blob/v8.4.0/build-logic/documentation/src/main/groovy/gradlebuild/docs/GradleJavadocsPlugin.java#L106-L109
• They could remove the element-list and only keep package-list if the docs have to be in HTML4 format
  • This element-list file being there is what misleads the new, smart javadoc implementation – if there is an element-list, then it calls readElementList(in, urlpath, false, 0, false), otherwise it does the same with a true as the last parameter (isOldFormDoc)
  • https://github.com/gradle/gradle/blob/v8.4.0/build-logic/documentation/src/main/groovy/gradlebuild/docs/GradleJavadocsPlugin.java#L113-L126