refactor: migrate alert_configuration resource and removing deprecated fields

[AgustinBettati]

Description

Ticket: INTMDB-996

Type of change:

• Bug fix (non-breaking change which fixes an issue). Please, add the "bug" label to the PR.
• New feature (non-breaking change which adds functionality). Please, add the "enhancement" label to the PR.
• Breaking change (fix or feature that would cause existing functionality to not work as expected). Please, add the "breaking change" label to the PR.
• This change requires a documentation update
• Documentation fix/enhancement

Required Checklist:

• I have signed the MongoDB CLA
• I have read the contribution guidelines
• I have added tests that prove my fix is effective or that my feature works per HashiCorp requirements
• I have added any necessary documentation (if appropriate)
• I have run make fmt and formatted my code

Further comments

• All acceptance tests are running, including the ones that require external credentials:
  
• I encountered some scenarios of failing non-empty plans in migrations tests, more details here: https://discuss.hashicorp.com/t/terraform-framework-migration-tests-with-non-empty-plan-due-to-optional-attributes/57305. (update) Solution: the optional attributes that could not be defined to null properly were set as computed, given that the api always responds with default values for them.

[andreaangiolillo]

LGTM. Thanks for addressing all the comments

[maastha]

LGTM