Rename merge request pipeline

Ref: #322682 (closed)

What does this MR do?

This MR renames our merge_request_pipeline method as merged_request_pipeline.

As brought up by @shinya.maeda:

We're having a inconsistency between Product (docs) and Engineering (codebase) in the Pipelines for merged result feature. Because of this, it's been confusing developers time to time and harming our productivity. We definitely want to use the single terminology for SSOT sake i.e. merged_result_pipeline? everywhere.

This is part of our effort to consolidate our naming with merged request pipeline.

Why are we doing this?

As our application keeps growing, it's important to refactor our codebase to better express what it actually does. Things change and something that could have made sense few months ago is not necessary true today.

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

cc @drew @fabiopitino