[AVM Module Issue]: Set default values to a secure value - SQL Managed Instances

### Module Name

avm/res/sql/managed-instance

### Description

All default values should comply with a security baseline, e.g. [NIST 800](https://docs.microsoft.com/en-us/azure/governance/policy/samples/nist-sp-800-53-r5)

The [build-in policies](https://github.com/Azure/azure-policy) of Azure can be used as a reference.

The task would be to scan over each of the following policies and make sure, that the module is per default complying to them.

The following policies are the NIST 800 ones:
\built-in-policies\policyDefinitions\SQL\SqlManagedInstance_AdvancedDataSecurity_Audit.json
\built-in-policies\policyDefinitions\SQL\SqlManagedInstance_EnsureServerTDEisEncryptedWithYourOwnKey_Audit.json
\built-in-policies\policyDefinitions\SQL\VulnerabilityAssessmentOnManagedInstance_Audit.json