Update CHANGELOG for Firestore v1.8.0 (#4296)

Author: wilhuff

Firestore/CHANGELOG.md

@@ -1,7 +1,11 @@
 # Unreleased
-- [changed] Removed the Firestore's dependency on `Protobuf`. If you're manually
-  tracking dependencies, you may be able to remove it from your build (note,
-  however, that other Firebase components may still require it).
+
+# v1.8.0
+- [changed] Removed Firestore's dependency on the `Protobuf` CocoaPod. If
+  you're manually tracking dependencies, you may be able to remove it from your
+  build (note, however, that other Firebase components may still require it).
+- [changed] Added a dependency on the `abseil` CocoaPod. If you're manually
+  tracking dependencies, you need to add it to your build.
 
 # v1.7.0
 - [feature] Added `whereField(_:in:)` and `whereField(_:arrayContainsAny:)` query