Generalise constraint on `instance Monoid (Maybe a)` to Semigroup

This now becomes possible due to the introduction of the

Semigroup=>Monoid superclass relation (see Trac #14191).

Reviewers: ekmett, RyanGlScott, austin, bgamari

Reviewed By: ekmett, RyanGlScott, bgamari

Subscribers: rwbarton, thomie

Differential Revision: https://phabricator.haskell.org/D3972