INTMBD-367: mongo_db_major_version attribute stale (resource: mongodbatlas_cluster)

[Zuhairahmed]

Description

Current default is version 5.0, not 4.4 when mongo_db_major_version is not specified. Also version 4.0 is now EOL. Updated documentation and linked to major version release notes.

Link to any related issue(s): https://jira.mongodb.org/browse/INTMDB-367

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
• [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
• [ ] I have added any necessary documentation (if appropriate)
• [ ] I have run make fmt and formatted my code

Further comments