concat2.t: Unskip an encoding.pm test

We should still test that it doesn’t crash, even if it
is deprecated.
subst.t: Rename test

This was copied and pasted from the previous test.
don't crash on deep recursion warnings in lexical subs (#118521)
Upgrade cv_flags_t from 16 to 32 bits.

Its main use is in a struct otherwise filled with pointers, which
means on 32-bit architectures its almost certainly taking up 32
bits anyway.
Manually add Utils delta for v5.19.1

I haven't fixed the adjustments to Porting/corelist.pl yet
for generating a utils delta.
Update release managers guide to note to bump Module::CoreList::Utils version too