[Release] Chandelog note about keychain access (#11496)

Author: ncooke3

FirebaseCore/CHANGELOG.md

@@ -1,3 +1,9 @@
+# Firebase 10.12.0
+- For developers building for visionOS, using products that use the Keychain
+  (e.g. FirebaseAuth) may fail to access the keychain on the visionOS
+  simulator. To workaround this, add the Keychain Sharing capability to the
+  visionOS target and explicitly add a keychain group (e.g. the bundle ID).
+
 # Firebase 10.11.0
 - [changed] Improved error reporting for misnamed configuration plist files (#11317).