[Feature Request] Update the usage of MySQL in the doc of Azure Spring Apps

[saragluna]

Context

With all the changes we made to support credential-free connections:

• https://github.com/Azure/azure-sdk-for-java/issues/29218
• https://github.com/Azure/azure-sdk-for-java/issues/29785
• https://github.com/Azure/azure-sdk-for-java/issues/28685

We should also update docs of Azure Spring Apps using MySQL to ensure they will use credential-free connections.

Goal

Update this doc https://docs.microsoft.com/en-us/azure/spring-apps/tutorial-managed-identities-mysql to include credential-free connections.