Bump version numbers: base-4.11.1.0, integer-gmp-1.0.2.0

This takes care of bumping the `base` and `integer-gmp`
minor version numbers in anticipation of a GHC 8.4.2 release.

While I was in town, I also filled in a `@since TODO` Haddock
annotation for `powModSecInteger` in `integer-gmp` with
`1.0.2.0`, and updated the changelog accordingly.

Test Plan: ./validate

Reviewers: hvr, goldfire, bgamari

Reviewed By: bgamari

Subscribers: thomie, carter

GHC Trac Issues: #15025

Differential Revision: https://phabricator.haskell.org/D4586