CONTRIB-4149 Add the phpcompatibility sniffs to moodle std.

Note this leads to some errors / warnings duplicated because
we have already the DeprecatedFunctionsSniff working, but it's
dependent of the PHP version being used, while the phpcompatibility
sniffs aren't. And better get some twice than miss them.
CONTRIB-4149 Update CHANGES.php with phpcompatibility info.