fix(magit): require Emacs 29.1+ for +forge

Forge depends on ghub, and ghub requires 29.1+, so Forge has dropped
backwards compatibility (plus, code-review depends on Forge).

Also adds a doctor check for this constraint.

Fix: #8419
Ref: magit/forge@35224e5dd4

modules/tools/magit/README.org

@@ -14,7 +14,8 @@ This module provides Magit, an interface to the Git version control system.
 ** Module flags
 - +forge ::
   Enable Forge; a porcelain for managing Github issues and PRs from within
-  Emacs. Will take a while on first run to build =emacsql-sqlite=.
+  Emacs. Will take a while on first run to build =emacsql-sqlite=. *Requires Emacs
+  29.1 or newer.*
 
 ** Packages
 - [[doom-package:evil-magit]] if [[doom-module::editor evil +everywhere]]

modules/tools/magit/packages.el

@@ -3,7 +3,10 @@
 
 (package! transient :pin "f3f498aa155f88c7e2ab6d1d01d1361813059db8") ; 0.9.2
 (package! magit :pin "2f1ff91f128f28aa277e0e060ef44b4be8a989c1") ; 4.3.6
-(when (modulep! +forge)
+(when (and (modulep! +forge)
+           ;; code-review depends on forge, and forge depends on ghub, which
+           ;; requires Emacs 29.1+
+           (version<= "29.1" emacs-version))
   (package! forge :pin "a31859547a1ea5e2acbab67b6b64f90134e2a156") ; 0.5.3
   (package! code-review
     :recipe (:host github