replaced 'Ability' with 'MongoAbility' as the default in CASL v6

PR Checklist

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• The commit message follows our guidelines: https://github.com/nestjs/docs.nestjs.com/blob/master/CONTRIBUTING.md

PR Type

What kind of change does this PR introduce?

• Bugfix
• Feature
• Code style update (formatting, local variables)
• Refactoring (no functional changes, no api changes)
• Build related changes
• Docs
• Other... Please describe:

What is the current behavior?

No current behavior to modify. This PR updates documentation.

Issue Number: #2596

What is the new behavior?

The documentation now reflects the change of using MongoAbility as the default in CASL v6 instead of Ability.

Does this PR introduce a breaking change?

• Yes
• No

Other information

In CASL v6, the MongoAbility class has become the default over the Ability class for better integration with MongoDB-based systems. This change reflects the evolving best practices and optimizations in CASL, allowing for more efficient querying and permission checks when working with MongoDB collections. By replacing Ability with MongoAbility, developers can leverage CASL's built-in MongoDB-specific capabilities, such as optimized querying and smoother integration with MongoDB's native operations.

For more details on how this change improves functionality, refer to the official CASL documentation: https://casl.js.org/