Build: Stop tracking branch tips for core submodules

Remove branch entries for MU_BASECORE, Common/MU, and
Common/MU_OEM_SAMPLE in .gitmodules so CI uses pinned submodule SHAs
instead of auto-advancing to the latest release/202511 branch tip.

Signed-off-by: Doug Flick <dougflick@microsoft.com>

Author: Flickdm

.gitmodules

@@ -1,15 +1,3 @@
-[submodule "MU_BASECORE"]
-	path = MU_BASECORE
-	url = https://github.com/microsoft/mu_basecore.git
-	branch = release/202511
-[submodule "Common/MU"]
-	path = Common/MU
-	url = https://github.com/microsoft/mu_plus.git
-	branch = release/202511
-[submodule "Common/MU_OEM_SAMPLE"]
-	path = Common/MU_OEM_SAMPLE
-	url = https://github.com/microsoft/mu_oem_sample.git
-	branch = release/202511
 [submodule "Silicon/Arm/TFA"]
 	path = Silicon/Arm/TFA
 	url = https://github.com/ARM-software/arm-trusted-firmware.git
@@ -33,3 +21,12 @@
 [submodule "Features/FFA"]
 	path = Features/FFA
 	url = https://github.com/microsoft/mu_feature_ffa.git
+[submodule "MU_BASECORE"]
+	path = MU_BASECORE
+	url = https://github.com/microsoft/mu_basecore.git
+[submodule "Common/MU"]
+	path = Common/MU
+	url = https://github.com/microsoft/mu_plus.git
+[submodule "Common/MU_OEM_SAMPLE"]
+	path = Common/MU_OEM_SAMPLE
+	url = https://github.com/microsoft/mu_oem_sample.git