perl5300delta: mention removal of arybase module

arybase was the internal module implementing the $[ feature. While the
removal of $[ is documented, the removal of arybase was not. This commit
tries to fix the oversight.

(Programmers were not supposed to use arybase directly, but it had its
own POD documentation and was officially listed as a core module in
perlmodlib, and previous perldeltas documented version changes in
arybase, so I feel its removal should be mentioned as well.)

See also GH #21619.

pod/perl5300delta.pod

@@ -667,6 +667,14 @@ not usually on concerns over their design.
 
 =item *
 
+arybase has been removed. It used to provide the implementation of the C<$[>
+variable (also known as the C<array_base> feature), letting array and string
+indices start at a non-zero value. As the feature has been removed (see
+L</Assigning non-zero to C<$[> is fatal>), this internal module is gone as
+well.
+
+=item *
+
 B::Debug is no longer distributed with the core distribution.  It
 continues to be available on CPAN as
 C<< L<B::Debug|https://metacpan.org/pod/B::Debug> >>.