update wording

[themantissa]

Description

Update wording - add deprecation message to whitelist resource.

Link to any related issue(s):
358

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
• Documentation fix/enhancement

Required Checklist:

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

Further comments

Likely will need a guide for users moving from old to new.

[themantissa]

@shum added the EoL for whitelist message (realized thanks to community the deprecation message was not here) and updated the main page - just need to check on wording. Thanks!