Persist target project id and target branch in merge trains table

What does this MR do?

Adding target_project_id and target_branch to merge_trains table.

Since we've not started persisting data yet (i.e. the count of rows on any instances is zero), we don't need to perform concurrent_* operations.

These columns will be used for further optimizations of merge trains related queries.

• CE: Here
• EE: https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/14060

https://gitlab.com/gitlab-org/gitlab-ee/issues/9186

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Performance 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

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