Java: Clarify supported language features

[igfoo]

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• docs/codeql/reusables/supported-versions-compilers.rst: Language not supported

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[smowton]

Will this be confusing re the difference between 7-22 (JVM versions that can execute the agent) vs 8-22 (versions whose language features we claim to support)? Strictly speaking that's true-- -source < 8 is silently treated as source 8 these days due to javac 22 having dropped the codepaths for the small number of scenarios where -source 7 programs are not forward-compatible with -source 8 -- but the vast majority of Java 1-1.7 programs are correctly understood, so perhaps we should just say "up to 22" at this granularity, only getting into the Java <= 7 issues in some higher-detailed doc, if ever (I don't believe we ever documented silently treating -source 1.6 as -source 7, for example).

[igfoo]

I considered that, but it would be a bit awkward if we got a bug report referencing our version claims from someone with a Java 7 class we analyse incorrectly.

I think it's OK to be precise in a footnote on a page that most people won't read carefully anyway.

[smowton]

We'll still get confused complaints from people asking if their -source 7 code is going to be rejected or what. How about adding "Older Java language versions are analysed as Java 8".