Revert "Merge branch 'mk/speed-up-merge-request-creation-action' into 'master'"

What does this MR do?

Reverts !57453 (merged) because it suffers from a multi-version compatibility problem in which Rails web and Sidekiq must be on the same version. If Rails web is ahead, then some tasks are not performed after MR creation. If Sidekiq is ahead, then some tasks are performed twice and I do not yet know the impact of that.

See !57453 (comment 544000274)

These are the tasks:

• Create todos for assignees, reviewers, and mentions
• Insert MergeRequestsClosingIssues associations
• Increment MR usage data counter
• Link LFS objects (if the MR source is a fork of the target)

After reverting, I can reintroduce the change but account for multi-version compatibility.

Does this MR meet the acceptance criteria?

Conformity

• 📋 Does this MR need a changelog?
  • I have included a changelog entry.
  • I have not included a changelog entry because _____.
• 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