[perl #89648] Remove ‘Perl folding rules...’ from dep cat

This warning message:

Perl folding rules are not up-to-date for 0x%x; please use the perlbug utility to report;

is in both ‘regexp’ and ‘deprecated’ warning categories. It is under
deprecated warnings so that it has a better chance of showing up.

I think this logic is faulty, because it’s not the deprecatedness that
makes a warning show up by default, but its defaultness; i.e., whether
Perl_ck_warner_d is used instead of Perl_ck_warner. The wide character
warning, for instance, is not under ‘deprecated’, but only ‘utf8’. Yet
it’s a default warning marked as S in perldiag. I think this message
should be treated similarly (and listed with (S regex)).

The next commit will update perldiag accordingly.
Move folding rules warning from 'D' to 'S'.

For RT #89648
diag.t: Handle ckWARN2reg_d properly
pod/perlvar.pod: Document changes to %ENV

Including Historical context, and relevance to IPC.

reformatted with Jim Keenans Suggestions
pod/perl5180delta.pod: Clarify %ENV Changes

Clarifies the slightly vague ( and  possibly wrong ) explanation for the change in %ENV behaviour.
checkAUTHORS.pl: Add alt addr for Kent Fredric
perlvar: Don’t mention dev version

Also, use the v convention used elsewhere in the same doc.
diagnostics.t: Adjust tests

They are sensitive to changes in perldiag.pod