Reduce minimum required git to 2.23

Thanks to raxod502/straight.el@7ca94a7, which was included in 3329448bd.
This in particular reduces the burden for Ubuntu users.

Relevant to #5088, #5096

README.md

@@ -98,7 +98,7 @@ Check out [the FAQ][FAQ] for answers to common questions about the project.
 
 
 # Prerequisites
-+ Git 2.28+
++ Git 2.23+
 + Emacs 26.3+ (*27.2 is recommended*, or [native-comp](https://www.emacswiki.org/emacs/GccEmacs)).
 + [ripgrep] 11.0+
 + GNU `find`

core/cli/doctor.el

@@ -71,10 +71,10 @@ in."
                (and (string-match "\\_<[0-9]+\\.[0-9]+\\(\\.[0-9]+\\)\\_>" version)
                     (match-string 0 version))))
          (if version
-             (when (version< version "2.28")
-               (error! "Git %s detected! Doom requires git 2.28 or newer!"
+             (when (version< version "2.23")
+               (error! "Git %s detected! Doom requires git 2.23 or newer!"
                        version))
-           (warn! "Cannot determine Git version. Doom requires git 2.28 or newer!")))))
+           (warn! "Cannot determine Git version. Doom requires git 2.23 or newer!")))))
 
    (unless (executable-find "rg")
      (error! "Couldn't find the `rg' binary; this a hard dependecy for Doom, file searches may not work at all")))

core/core-packages.el

@@ -132,8 +132,8 @@ uses a straight or package.el command directly).")
                 (and (string-match "\\_<[0-9]+\\.[0-9]+\\(\\.[0-9]+\\)\\_>" version)
                      (match-string 0 version))))
           (if version
-              (when (version< version "2.28")
-                (user-error "Git %s detected! Doom requires git 2.28 or newer!"
+              (when (version< version "2.23")
+                (user-error "Git %s detected! Doom requires git 2.23 or newer!"
                             version)))))
       (print! (start "Installing straight..."))
       (print-group!
@@ -159,10 +159,8 @@ uses a straight or package.el command directly).")
                             "--branch" straight-repository-branch))
                (make-directory repo-dir 'recursive)
                (let ((default-directory repo-dir))
-                 ;; git init's -b switch was introduced in 2.28. As much as I'd
-                 ;; like to, the dependency is unavoidable because straight.el
-                 ;; uses it internally.
-                 (funcall call "git" "init" "-b" straight-repository-branch)
+                 (funcall call "git" "init")
+                 (funcall call "git" "branch" "-m" straight-repository-branch)
                  (funcall call "git" "remote" "add" "origin" repo-url
                           "--master" straight-repository-branch)
                  (funcall call "git" "fetch" "origin" pin

docs/getting_started.org

@@ -96,7 +96,7 @@ us know!
 * Install
 This is what you'll have installed by the end of this section:
 
-- Git 2.28+
+- Git 2.23+
 - Emacs 26.3+ *(27.2 is recommended, or [[https://www.emacswiki.org/emacs/GccEmacs][native-comp]])*
 - [[https://github.com/BurntSushi/ripgrep][ripgrep]] 11.0+
 - GNU Find
@@ -119,24 +119,8 @@ In the unusual case that Emacs 26.x is unavailable through your package manager,
 you'll have to [[https://www.gnu.org/software/emacs/manual/html_node/efaq/Installing-Emacs.html][build it from source]], but here are installation instructions for many popular Linux distributions:
 
 **** Ubuntu
-There are two challenges for Ubuntu users:
-
-+ Ubuntu <= 18.04 only provide Emacs <= 25.3 (Doom requires 26.3+)
-+ Ubuntu <= 20.04 only provide git <= 2.25.1 (Doom requires 2.28+)
-
-  To get around these:
-
-***** Git
-To install Git 2.28:
-
-#+BEGIN_SRC bash
-add-apt-repository ppa:git-core/ppa
-apt update
-apt install git
-#+END_SRC
-
-***** Emacs
-To install Emacs 27, you'll need either a PPA or Snap:
+Emacs 27.x is not available through Ubuntu's package manager out-of-the-box. To
+get around this you'll need to install Emacs through a PPA:
 
 #+BEGIN_SRC bash
 add-apt-repository ppa:kelleyk/emacs