Add Ci::StuckBuilds::DropService for use in StuckCiJobsWorker

What does this MR do?

This MR moves logic out of the StuckCiBuildsWorker and into a DropService that can be utilized by many separate workers. This is the first step in an effort #334399 (closed) to eventually move the queries for stuck builds out into separate workers, by build status, to improve reliability in the event that one query times out or fails.

The plans are largely laid out in !64635 (closed).

How to setup and validate locally (strongly suggested)

There is no outward-facing behavioral change in this merge request. To validate that it works, see the specs that have been moved directly into the DropService spec file from the StuckCiJobsWorker spec file, without substantial changes.

To be sure of this, the logic was moved in the first commit without any spec modification, beyond a some reference changes due to methods and constants that are no longer defined in the same place. All of the specs remain in place there, all passing.

In the second commit, we move the specs over without changing any application logic. Here, the behaviors similarly remain the same with no failures.

The third commit merely adds some assertions about DropService#execute implementation details to shore up our confidence as we change the behavior. Those specs will need to be changed in order to have the service execute on a per-status basis, so those assertions will need to be changed manually and explicitly reviewed in the diffs of merge requests that alter the behavior.

Does this MR meet the acceptance criteria?

Conformity

• I have included changelog trailers, or none are needed. (Does this MR need a changelog?)
  • These changes are entirely developer-facing.
• I have added/updated documentation, or it's not needed. (Is documentation required?)
  • These changes are entirely developer-facing.
• I have properly separated EE content from FOSS, or this MR is FOSS only. (Where should EE code go?)
• I have added information for database reviewers in the MR description, or it's not needed. (Does this MR have database related changes?)
  • There are no querying modifications here.
• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.
• This change is backwards compatible across updates, or this does not apply.

Availability and Testing

• I have added/updated tests following the Testing Guide, or it's not needed. (Consider all test levels. See the Test Planning Process.)
• I have tested this MR in all supported browsers, or it's not needed.
• I have informed the Infrastructure department of a default or new setting change per definition of done, or it's not needed.

Security

Does this MR contain changes to processing or storing of credentials or tokens, authorization and authentication methods or other items described in the security review guidelines? If not, then delete this Security section.

• 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