Fix NameError: uninitialized constant ActiveRecord::ConnectionAdapters::Mysql2::DatabaseStatements::ER_UNKNOWN_STMT_HANDLER

https://buildkite.com/rails/rails/builds/115365#01943d81-dfd3-49c1-8a01-68ba87e6fe3f/1226-1237

Follow up to 6d5633f.

Author: kamipo

activerecord/lib/active_record/connection_adapters/mysql2/database_statements.rb

@@ -70,7 +70,7 @@ def perform_query(raw_connection, sql, binds, type_casted_binds, prepare:, notif
                 # but the client doesn't know it.
                 # But we know that this error is safe to retry, so we do so after
                 # getting rid of the originally cached statement.
-                if error.error_number == ER_UNKNOWN_STMT_HANDLER
+                if error.error_number == Mysql2Adapter::ER_UNKNOWN_STMT_HANDLER
                   if retry_count.positive?
                     retry_count -= 1
                     retry