docs: Add missing compat annotation for isdefinedglobal (#58542)

Add missing compatibility annotation for `isdefinedglobal`. Fixes
#58528.

(cherry picked from commit 953903b)

Author: Keno

base/docs/basedocs.jl

@@ -2796,6 +2796,9 @@ a value set.
 If `allow_import` is `false`, the global variable must be defined inside `m`
 and may not be imported from another module.
 
+!!! compat "Julia 1.12"
+    This function requires Julia 1.12 or later.
+
 See also [`@isdefined`](@ref).
 
 # Examples