Drop the 5 (implement PPC0025)

[book]

This is a draft MR, to test how things would actually work.

Many of the remaining failing tests are dual-life modules (which perform a string comparison againsg $]), or misplaced expectations (use v6 should now work, and enable the :5.41 feature and builtin bundles).

• This set of changes requires a perldelta entry, and I need help writing it.