gitmodules: Fix tracking branch for GCC 12

A recent commit bumped the GCC version to 12.2,
but adjusting the tracking branch in .gitmodules was forgotten.
Let's fix this and set the tracking branch to gcc-12
(note, that there are no release branches for minor version releases).

Reported-by: Tommy Murphy <tommy_murphy@hotmail.com>
Signed-off-by: Christoph Müllner <christoph.muellner@vrull.eu>

.gitmodules

@@ -5,7 +5,7 @@
 [submodule "gcc"]
 	path = gcc
 	url = https://gcc.gnu.org/git/gcc.git
-	branch = releases/gcc-12.1.0
+	branch = releases/gcc-12
 [submodule "glibc"]
 	path = glibc
 	url = https://sourceware.org/git/glibc.git