[Documentation Update]: Make AzAPI the default provider for AVM

[terrymandin]

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Description

The AVM specification should be updated to make AzAPI the default provider instead of AzureRM for the following reasons:

1. AzAPI allows preview features
2. New features are available as soon as they are available in the APIs
3. We are not dependent on another team when new features are released
4. It is easier to retire AzureRM if AVM is the Terraform approach going forward

[microsoft-github-policy-service]

Important

The "Needs: Triage 🔍" label must be removed once the triage process is complete!

Tip

For additional guidance on how to triage this issue/PR, see the AVM Issue Triage documentation.

[terrymandin]

Note that there is a workaround for allowing preview features by using azapi_update_resource that can be run after the AzureRM call.

[matt-FFFFFF]

We have discussed internally and will make recommendations prior to v1.0 of AVM