Remove explicit cast from index condition

Remove an explicit cast from an index condition which casts the compared
value to `timestamp with time zone`. Some installations could have the
`created_at` column as a `timestamp without time zone`, which causes
index creation to fail.

By removing the explicit cast, postgres casts the timestamp expression
to the appropriate timestamp type.

db/post_migrate/20211103162025_add_index_on_events_using_btree_created_at_id.rb

@@ -4,7 +4,7 @@ class AddIndexOnEventsUsingBtreeCreatedAtId < Gitlab::Database::Migration[1.0]
   INDEX_NAME = 'index_events_on_created_at_and_id'
   TABLE = :events
   COLUMNS = %i[created_at id]
-  CONSTRAINTS = "created_at > '2021-08-27 00:00:00+00'::timestamp with time zone"
+  CONSTRAINTS = "created_at > '2021-08-27 00:00:00+00'"
   disable_ddl_transaction!
 
   def up