use IF EXISTS with ALTER USER to avoid replication errors

[samongyr-sq]

Example replication error

root@songtest-aws-development-aurora-use1-314 (none)> select * from performance_schema.replication_applier_status_by_worker\G
*************************** 1. row ***************************
                                           CHANNEL_NAME:
                                              WORKER_ID: 1
                                              THREAD_ID: NULL
                                          SERVICE_STATE: OFF
                                      LAST_ERROR_NUMBER: 1396
                                     LAST_ERROR_MESSAGE: Worker 1 failed executing transaction '252a6a94-3c4c-3ade-a3ce-991d1e2fc76a:3' at source log mysql-bin-changelog.000003, end_log_pos 988; Error 'Operation ALTER USER failed for 'app'@'%'' on query. Default database: ''. Query: 'ALTER USER 'app'@'%' IDENTIFIED WITH 'mysql_native_password' AS '*581853763E56537FEE83984674AA4B4945BE6EEC''
                                   LAST_ERROR_TIMESTAMP: 2024-12-03 20:24:00.071630
                               LAST_APPLIED_TRANSACTION:
     LAST_APPLIED_TRANSACTION_ORIGINAL_COMMIT_TIMESTAMP: 0000-00-00 00:00:00.000000
    LAST_APPLIED_TRANSACTION_IMMEDIATE_COMMIT_TIMESTAMP: 0000-00-00 00:00:00.000000
         LAST_APPLIED_TRANSACTION_START_APPLY_TIMESTAMP: 0000-00-00 00:00:00.000000
           LAST_APPLIED_TRANSACTION_END_APPLY_TIMESTAMP: 0000-00-00 00:00:00.000000
                                   APPLYING_TRANSACTION: 252a6a94-3c4c-3ade-a3ce-991d1e2fc76a:3
         APPLYING_TRANSACTION_ORIGINAL_COMMIT_TIMESTAMP: 2024-12-03 05:07:19.556002
        APPLYING_TRANSACTION_IMMEDIATE_COMMIT_TIMESTAMP: 2024-12-03 05:07:19.556002
             APPLYING_TRANSACTION_START_APPLY_TIMESTAMP: 0000-00-00 00:00:00.000000
                 LAST_APPLIED_TRANSACTION_RETRIES_COUNT: 0
   LAST_APPLIED_TRANSACTION_LAST_TRANSIENT_ERROR_NUMBER: 0
  LAST_APPLIED_TRANSACTION_LAST_TRANSIENT_ERROR_MESSAGE:
LAST_APPLIED_TRANSACTION_LAST_TRANSIENT_ERROR_TIMESTAMP: 0000-00-00 00:00:00.000000
                     APPLYING_TRANSACTION_RETRIES_COUNT: 0
       APPLYING_TRANSACTION_LAST_TRANSIENT_ERROR_NUMBER: 0
      APPLYING_TRANSACTION_LAST_TRANSIENT_ERROR_MESSAGE:
    APPLYING_TRANSACTION_LAST_TRANSIENT_ERROR_TIMESTAMP: 0000-00-00 00:00:00.000000
    ```