Azure Database for MySQL single server is on the retirement path

[nasaldrops]

Resolves #https://github.com/microsoft/AzureTRE/issues/3540

What is being addressed

Single Server is retiring in favour of Flexible server so updating azurerm_mysql_server to azurerm_mysql_flexible_server

How is this addressed

• Update terraform files referencing azurerm_mysql__server to azurerm_mysql_flexible_server.
• sku_name changes

[github-actions[bot]]

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