Update Wiki URLs to point to GitLab

This moves all URL references to Trac Wiki to their corresponding GitLab counterparts.

This substitution is classified as follows:

1. Automated substitution using sed with Ben's mapping rule:

   • Old: ghc.haskell.org/trac/ghc/wiki/XxxYyy...
   • New: gitlab.haskell.org/ghc/ghc/wikis/xxx-yyy...

2. Manual substitution for URLs containing # index:

   • Old: ghc.haskell.org/trac/ghc/wiki/XxxYyy...#Zzz
   • New: gitlab.haskell.org/ghc/ghc/wikis/xxx-yyy...#zzz

3. Manual substitution for strings starting with Commentary:

   • Old: Commentary/XxxYyy...
   • New: commentary/xxx-yyy...

See also !539 (merged).