doc: Update 1.12.0 release guide and Changelog

[maastha]

Description

This PR updates the Changelog and release guide for v1.12.0 of the provider to include guidance for this fix #1471

Link to any related issue(s):

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
• If changes include deprecations or removals, I defined an isolated PR with a relevant title as it will be used in the auto-generated changelog.

Further comments

[Zuhairahmed]

Thanks for this @maastha! Made an additional commit and one suggestion. Also suggest let's wait to hear back from NTSE in case of any additional feedback before releasing v1.12.1. If nothing by tomorrow your time feel free to release.

[jwilliams-mongo]

@maastha LGTM % a few style tweaks + moving Breaking Changes earlier in that item

[jwilliams-mongo]

I think breaking change should be the first thing in this item. I think we should also bold this text to make it stand out. Also a minor wording tweak to make the structure of this item align with the others.

Suggested change

	- Fix when removing `collectionName` from user role [\#1471](https://github.com/mongodb/terraform-provider-mongodbatlas/pull/1471) [Breaking Change] ([marcosuma](https://github.com/marcosuma))
	- **Breaking Change**: Fixes an issue where removing `collectionName` from user role didn't work [\#1471](https://github.com/mongodb/terraform-provider-mongodbatlas/pull/1471) ([marcosuma](https://github.com/marcosuma))