Upgrade build requirement to Java 13?

[Martin Desruisseaux]

Hello all

SIS currently requires Java 11 for building, but the compilation result 
is executable on Java 8. Is there objection to upgrade the build 
requirement to Java 13? The compilation result however would continue to 
target Java 8 for now.

The reason is that there is some minor changes in number formatting, 
with 'NO-BREAK SPACE' (U+00A0) replaced by 'NARROW NO-BREAK SPACE' 
(U+202F) in French locale. Requesting the latest Java version at 
compile-time avoid the need to complexify a JUnit test by making it 
tolerant to 2 Java versions.

    Martin