refactor!(vc-gutter): drop git-gutter for diff-hl

BREAKING CHANGE: This removes git-gutter as an implementation for the
`:ui vc-gutter` module, leaving only the diff-hl implementation. There
are no longer any +git-gutter or +diff-hl flags for this module. Users
don't have to do anything to keep the vc gutter, unless they prefer
git-gutter for any reason (in which case they'll need to install and set
it up themselves).

This has been planned for some time, because of a roadmap goal for Doom
to lean into native/built-in functionality where it's equal or better
than the third party alternatives. diff-hl relies on the built-in vc.el
library instead of talking to git directly (thus expanding support to
whatever VCS's vc.el supports, and not git alone), which also means it
can take advantage of its caching and other user configuration for
vc.el. Overall, it is faster and lighter.

What I've also been waiting for was a stage-hunk command, similar to
git-gutter:stage-hunk, which arrived in dgutov/diff-hl@a0560551cd and
dgutov/diff-hl@133538973b, and have evolved since.

Ref: dgutov/diff-hl@a0560551cd
Ref: dgutov/diff-hl@133538973b
Ref: https://github.com/orgs/doomemacs/projects/5/views/1?pane=issue&itemId=58747789

lisp/doom-keybinds.el

@@ -82,7 +82,7 @@ and Emacs states, and for non-evil users.")
 ;; 1. Quit active states; e.g. highlights, searches, snippets, iedit,
 ;;    multiple-cursors, recording macros, etc.
 ;; 2. Close popup windows remotely (if it is allowed to)
-;; 3. Refresh buffer indicators, like git-gutter and flycheck
+;; 3. Refresh buffer indicators, like diff-hl and flycheck
 ;; 4. Or fall back to `keyboard-quit'
 ;;
 ;; And it should do these things incrementally, rather than all at once. And it