chore: Fix Atlas variables

[lantoli]

Proposed changes

Jira ticket: INTMDB-1169

Fix Atlas variables to use consistently: MONGODB_ATLAS_ORG_ID, MONGODB_ATLAS_PUBLIC_KEY, MONGODB_ATLAS_PUBLIC_KEY, MONGODB_ATLAS_BASE_URL instead of ATLAS_ORG_ID, etc.

Type of change:

• 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

Manual QA performed:

• cfn invoke for each of CRUDL/cfn test
• Updated resource in example
• Published to AWS private registry
• Used the template in example to create and update a stack in AWS
• Deleted stack to ensure resources are deleted
• Created multiple resources in same stack
• Validated in Atlas UI
• Included screenshots

Required Checklist:

• I have signed the MongoDB CLA
• I have added tests that prove my fix is effective or that my feature works
• I have added any necessary documentation (if appropriate)
• I have run make fmt and formatted my code
• For CFN Resources: I have released by changes in the private registry and proved by change
  works in Atlas

Further comments