Acctest - fix mysql RP test case failures

[sinbai]

PR fixes the following test case failures in Teamcity:

1. TestAccMySqlFlexibleServer_pitr and TestAccMySqlFlexibleServer_updateReplicationRole fail with the following error, ignore_changes source_server_id because API does not return its value when create_mode is PointInTimeRestore/Replica.
   

Refer to: PR #27295, PR #27442.

2. TestAccDataSourceMySqlFlexibleServer_basic fails with the following error, ignore_changes zone because API returns a value for it when it is not specified.
   

3. Some test cases failied with the following ZoneNotAvailableForRegion error, this is because the Available Zone of eastus2 has changed from 1 and 2 to 2, 3.
   

4. Some test cases fail with the following 500 Internal Server Error, per this document, the value B_Standard_B1s of sku_name is currently not supported, change to B_Standard_B1ms instead of B_Standard_B1s to fix tests failure .
   

5. Fix test case TestAccMySqlFlexibleServer_updateMaintenanceWindow fails with the following issues.
   

Note: Remove zone and ignore change zone since the zone was removed from func basic in PR, which caused error "zone cannot be changed independently". Given that removing zone from func basic is correct, and Maintenance Window testing does not require zone to be specified, so remove zone and ignore change zone to fix the failure.

Test Results:
https://hashicorp.teamcity.com/buildConfiguration/TF_AzureRM_AZURERM_SERVICE_PUBLIC_MYSQL?branch=refs%2Fpull%2F28882%2Fmerge&buildTypeTab=overview