add: remove "add.interactive.useBuiltin" & Perl "git add--interactive"

Since [1] first released with Git v2.37.0 the built-in version of "add
-i" has been the default. That built-in implementation was added in
[2], first released with Git v2.25.0.

At this point enough time has passed to allow for finding any
remaining bugs in this new implementation, so let's remove the
fallback code.

As with similar migrations for "stash"[3] and "rebase"[4] we're
keeping a mention of "add.interactive.useBuiltin" in the
documentation, but adding a warning() to notify any outstanding users
that the built-in is now the default. As with [5] and [6] we should
follow-up in the future and eventually remove that warning.

1. 0527ccb1b55 (add -i: default to the built-in implementation,
   2021-11-30)
2. f83dff60a78 (Start to implement a built-in version of `git add
   --interactive`, 2019-11-13)
3. 8a2cd3f5123 (stash: remove the stash.useBuiltin setting,
   2020-03-03)
4. d03ebd411c6 (rebase: remove the rebase.useBuiltin setting,
   2019-03-18)
5. deeaf5ee077 (stash: remove documentation for `stash.useBuiltin`,
   2022-01-27)
6. 9bcde4d5314 (rebase: remove transitory rebase.useBuiltin setting &
   env, 2021-03-23)

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 1 insertions, 1 deletions

diff --git a/INSTALL b/INSTALL
index d5694f8c47..4b42288882 100644
--- a/INSTALL
+++ b/INSTALL
@@ -120,7 +120,7 @@ Issues of note:
 	  for everyday use (e.g. "bisect", "request-pull").
 
 	- "Perl" version 5.8 or later is needed to use some of the
-	  features (e.g. preparing a partial commit using "git add -i/-p",
+	  features (e.g. sending patches using "git send-email",
 	  interacting with svn repositories with "git svn").  If you can
 	  live without these, use NO_PERL.  Note that recent releases of
 	  Redhat/Fedora are reported to ship Perl binary package with some