Clarify Java prerequisites and support in Readme

[kwin]

Currently it is hard to figure out which version requires (for running) and supports (for formatting) which Java version. Would be nice to add a table with the lowest supported Java runtime version and the highest supported Java source version (i.e. which syntax features introduced in a certain JLS version are supported).

[cushon]

It would be good to document this.

The basic idea is that for google-java-format to format a particular Java language version, it needs to be running on a JDK version that is new enough that the JDK's javac version could compile the code.

i.e. to format Java 21, it needs to be running on JDK 21 or newer.

[cushon]

This was fixed by @vorburger in 14ae0a7

[kwin]

I think this covers only parts of it. The newest version requires java 17 no matter which version the source file is in. That deserves being mentioned as well.

[vorburger]

@kwin Hi! (It's a small world...) How about #1235?

[kwin]

@vorburger Great, thanks. Yes, that helps a lot!