[Microsoft.DBforMySQL/flexibleServers/configurations]: `properties` shouldn't be marked as read-only?

### Resource Type

Microsoft.DBforMySQL/flexibleServers/configurations

### Api Version

2021-05-01

### Issue Type

Property(s) inaccurately marked read-only/write-only

### Other Notes

The following warnings are generated by the Bicep linter:

`Warning BCP245: Resource type "Microsoft.DBforMySQL/flexibleServers/configurations@2021-05-01" can only be used with the 'existing' keyword.`

`Warning BCP073: The property "properties" is read-only. Expressions cannot be assigned to read-only properties.`

However, I am able to declare the type in question as a regular resource and successfully update MySQL Flexible Server parameters through it.

### Bicep Repro

_No response_

### Confirm

- [X] I have searched this repository and have not found similar issue reports.

Start with the Microsoft.DBforMySQL/flexibleServers/configurations resource type at API version 2021-05-01 and inspect why `properties` is marked read-only. Verify the behavior against the reported Bicep linter warnings and the ability to update MySQL Flexible Server parameters; done means a regular resource declaration accepts the update without those warnings.

Written by the indexing model from the issue text.