PoC: Simplify fixed pipeline email notification

What does this MR do?

This MR is a showcase of a proposal to simplify !16951 (merged).

The ideas behind it are:

• We don't use ci_refs table, but dynamically calculate the status transition. Design on ci_refs table requires architectural discussion and likely take more time, so for now, we calculate it without persisting it. The calculation itself is very cheap, so there are no performance penalty.
• Ci::Pipeline#status_transition_on_same_context takes into account of any kind of pipelines, not only branch pipelines, but also MR pipelines.
• We calculate the transition status in PipelineNotificationWorker that is invoked when a pipeline finished. This means the calculation happens in sidekiq worker.
• This resolves all of the concerns in !16951 (merged).

Issue is #24309 (closed).

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