Added missing changelogs (#877)

Co-authored-by: Aidan Haran <aharan@fusioneer.com>

Author: aidanharan

CHANGELOG.md

@@ -3,6 +3,8 @@
 #### Fixed
 
 - [#872](https://github.com/rails-sqlserver/activerecord-sqlserver-adapter/pull/872) Use native String#start_with
+- [#873](https://github.com/rails-sqlserver/activerecord-sqlserver-adapter/pull/873) Various fixes to get the tests running for Rails 6.1
+- [#874](https://github.com/rails-sqlserver/activerecord-sqlserver-adapter/pull/874) Deduplicate schema cache structures
 
 #### Changed