doc: Remove stale references to EOL MongoDB versions in registry docs - #4790

Description

Removes outdated references to older MongoDB versions (4.2, 4.4, 6.0, 7.0) from the Terraform Registry documentation. The removed caveats for 6.0 and earlier are satisfied by every MongoDB version Atlas currently supports (7.0 and later), and the 7.0-specific OIDC note no longer adds value now that MongoDB 9.0 is GA.

Changes per file:

• docs/resources/database_user.md, docs/data-sources/database_user.md, docs/data-sources/database_users.md — Replaced the "OIDC support is only available starting in MongoDB 7.0" note (which linked to a retired marketing page) with a plain pointer to the Atlas OIDC documentation.
• docs/resources/custom_db_role.md — Updated the privilege-action compatibility example from MongoDB 4.2/4.4 to 8.0/9.0.
• docs/resources/advanced_cluster.md, docs/data-sources/advanced_cluster.md, docs/data-sources/advanced_clusters.md — Removed the "MongoDB 6.0" qualifier from the default_write_concern default, deleted the "only supported for MongoDB version 6.0 and above" caveat from change_stream_options_pre_and_post_images_expire_after_seconds, and dropped a duplicated write-concern link.
• docs/resources/global_cluster_config.md, docs/data-sources/global_cluster_config.md — Removed the "only available for Atlas clusters with MongoDB v4.4 and later" caveat from is_custom_shard_key_hashed.

Docs for the deprecated mongodbatlas_cluster resource/data sources and historical upgrade guides are intentionally left unchanged.

Link to any related issue(s): DOCSP-60146

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. A migration guide must be created or updated if the new feature will go in a major version.
• Breaking change (fix or feature that would cause existing functionality to not work as expected). Please, add the "breaking change" label to the PR. A migration guide must be created or updated.
• This change requires a documentation update
• Documentation fix/enhancement

Required Checklist:

• I have signed the MongoDB CLA
• I have read the contributing guides
• I have checked that this change does not generate any credentials and that they are NOT accidentally logged anywhere.
• 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 fix and verified my code
• If changes include deprecations or removals I have added appropriate changelog entries.
• If changes include removal or addition of 3rd party GitHub actions, I updated our internal document. Reach out to the APIx Integration slack channel to get access to the internal document.

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