Fix merge request GraphQL resolver

What does this MR do?

The Merge Requests query doesn't respond to pagination params such as first. Additionally, the resolver returns merge requests that belong to all projects.

• use the Batchloader to fetch MRs when iids are not passed as params to the query and the Merge Requests finder otherwise
• add specs to test that pagination works e.g when first is passed as a parameter

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• [-] Merge request performance guidelines
• [-] Style guides
• [-] Database guides
• [-] Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers
• [-] Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• [-] Label as security and @ mention @gitlab-com/gl-security/appsec
• [-] The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• [-] Security reports checked/validated by a reviewer from the AppSec team

#34527 (closed)