Update CHANGELOG for Firestore v1.3.1 (#2994)

Author: wilhuff

Firestore/CHANGELOG.md

@@ -1,5 +1,13 @@
 # Unreleased
 
+# 1.3.1
+- [fixed] Disabling garbage collection now avoids even scheduling the
+  collection process. This can be used to prevent crashes in the background when
+  using `NSFileProtectionComplete`. Note that Firestore does not support
+  operating in this mode--nearly all API calls will cause crashes while file
+  protection is enabled. This change just prevents a crash when Firestore is
+  idle (#2846).
+
 # 1.3.0
 - [feature] You can now query across all collections in your database with a
   given collection ID using the `Firestore.collectionGroup()` method.