STY: Adopt three-letter prefix for maintenance commits

Adopt three-letter prefix for maintenance commits: use `MNT` instead of
`MAINT`:
- Less typing; still readable
- All other prefixes are three-letter prefixes except for `CI`
- There has been a slight shift towards `MNT` lately, e.g.
  https://github.com/nipreps/fmriprep/blob/f3464b5cd4991042be18d131cbf3ed9c8dd413d9/CHANGES.rst?plain=1#L65;
  https://github.com/nipreps/smriprep/blob/b839befd008e02f329c59aa634583a18e4049829/CHANGES.rst?plain=1#L31;
  https://github.com/nipreps/nireports/blob/968d98f7774fafbc745f9d5eb73b40d1e7be4e4c/CHANGES.rst?plain=1#L34;
  https://github.com/nipreps/sdcflows/blob/b4a01f6e2071a538896e228cba9a43c10b8256cb/CHANGES.rst?plain=1#L34

Author: jhlegarreta

docs/community/CONTRIBUTING.md

@@ -233,7 +233,7 @@ but those accepted fastest will follow a workflow similar to the following:
      * `STY`: style changes ([example][sty_ex])
      * `REF`: refactoring existing code ([example][ref_ex])
      * `CI`: updates to continuous integration infrastructure ([example][ci_ex])
-     * `MAINT`: general maintenance ([example][maint_ex])
+     * `MNT`: general maintenance ([example][maint_ex])
      * For works-in-progress, add the `WIP` tag in addition to the descriptive prefix.
        Pull-requests tagged with `WIP:` will not be merged until the tag is removed.