[MySql] Merge Update-AzMySqlServer & Update-AzMySqlServerConfiguration

## Description of the new feature
It was not clear for the users what was the purpose of ‘Update-MySqlServer’ and ‘Update-MySqlServerConfiguration’. The documentation could potentially fix this along a potential change in the cmdlet name.

## Proposed implementation details (optional)
One solution is to merge these 2 cmdlets. Need further discussion.

Start by locating the Update-AzMySqlServer and Update-AzMySqlServerConfiguration cmdlet definitions and their documentation. Clarify whether the cmdlets should be merged or renamed, then confirm the intended scope and acceptance criteria with maintainers before changing either interface.

Written by the indexing model from the issue text.