[core] Remove NumberFormatException from innermost try/catch in parseAndProcessStanza()
This was added in 02d8f53246b3 ("[core] Increase resilience against
faulty parser implementions"). However, NumberFormatException is a
subclass of IllegalArgumentException, which is already catched by the
outermost catch.
[build-logic] Remove some isJavaXCompatible blocks
Since Smack requires now Java 11, all conditional code for lower Java
versions can be dropped from the build logic.
