[Release] Update FIAM changelog to reflect 10.26.0

Author: ncooke3

FirebaseInAppMessaging/CHANGELOG.md

@@ -1,6 +1,9 @@
 # Unreleased
-- [fixed] Fixed crash at app start that affected SwiftPM users and CocoaPods
-  users using static frameworks (#12882).
+- [fixed] Fixed crash at app start that affected CocoaPods users using static
+  frameworks (#12882).
+
+# 10.26.0
+- [fixed] Fixed crash at app start that affected SwiftPM users (#12882).
 
 # 10.25.0
 - [changed] Removed usages of user defaults API to eliminate required reason