Replace <+> with <>

[johnli360]

Description

Replaces <+> usage with <> as discussed in https://github.com/xmonad/xmonad/issues/132#issuecomment-886218490

Checklist

• [x] I've read CONTRIBUTING.md

• [x] I've considered how to best test these changes (property, unit, manually, ...) and concluded: I ran cabal build using ghc 8.4.4, and 9.2.2

• [x] I updated the CHANGES.md file

[slotThe]

Thank you!