[Infra] Add changelog entry for Xcode 26 crash fix (#15166)

Author: ncooke3

FirebaseInAppMessaging/CHANGELOG.md

@@ -1,3 +1,8 @@
+# 12.1.0
+- [fixed] Fix Xcode 26 crash from missing `NSUserActivityTypeBrowsingWeb`
+  symbol. Note that this fix isn't in the 12.1.0 zip and Carthage
+  distributions, but will be included from 12.2.0 onwards. (#15159)
+
 # 11.0.0
 - [removed] **Breaking change**: The deprecated `FirebaseInAppMessagingSwift`
   module has been removed. See