Relying on atlas api for unit validation on alert configuration

[evertsd]

Description

Update alert configuration to defer unit validation atlas api. An alternative solution would've been to update the ValidateFunc to support the following values

• NANOSECONDS
• MILLION_MINUTES
• RPU
• THOUSAND_RPU
• MILLION_RPU
• WPU
• THOUSAND_WPU
• MILLION_WPU

The above values were gathered from the following documentation under metricThreshold.units https://www.mongodb.com/docs/atlas/reference/api/alert-configurations-get-config/#request-path-parameters

Closes #854

Type of change:

• [x] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
• [ ] This change requires a documentation update
• [x] Documentation fix/enhancement

Required Checklist:

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

Further comments