Fix javadoc generation broken on JDK 20

Fix javadoc generation broken on JDK 20

Firstly, Javadoc 20 cannot handle source level 1.7 anymore, so I bumped
it to 11, which hopefully will be around for another while. It also
matches the ajc minimum of Java 11, i.e. ajdoc and ajc are synchronised
in this regard now.

Secondly, as of JDK 20, the javadoc tool renders HTML pages with section
headers specific to the JVM default locale. The workaround is to set the
locale to English before calling javadoc.

Signed-off-by: Alexander Kriegisch <Alexander@Kriegisch.name>