Only block pausing indexing if running migration requires it

What does this MR do?

Related to #329273 (closed)

Advanced Search migrations have the option to require indexing to be paused. Currently, the Advanced Search settings page will disable the Pause Elasticsearch indexing checkbox if there are ANY pending migrations and indexing is paused. This has created some problems in the past if users pause indexing before migrations run.

This change updates the logic used to disable the checkbox to check that:

• a pending migration existing
• the first pending migration (only 1 migration can run at a time)
  • is running (started, not completed, and not halted)
  • requires indexing to be paused

How to test

Note: Make sure Elasticsearch is setup in gdk, Advanced Search is enabled and the projects/data are indexed.

1. Navigate to the Admin Settings - Settings - Advanced Search Settings screen
2. Pause Elasticsearch indexing, save, and verify that Pause Elasticsearch indexing is checked
3. Create a new migration file that does not require indexing to be paused under /ee/elastic/migrate (see examples below for what I used)
4. Open rails console
5. Verify there are pending migrations: Elastic::DataMigrationService.pending_migrations?
6. Start a Migration Worker Elastic::MigrationWorker.perform_async
7. Refresh the Advanced Search Settings page and verify that Pause Elasticsearch indexing is checked and enabled
8. change the migration completed? to return true
9. Start a Migration Worker Elastic::MigrationWorker.perform_async
10. Refresh the Advanced Search Settings page and verify that Pause Elasticsearch indexing is checked and enabled
11. Create a new migration file that requires indexing to be paused under /ee/elastic/migrate (see examples below for what I used)
12. Verify there are pending migrations: Elastic::DataMigrationService.pending_migrations?
13. Start a Migration Worker Elastic::MigrationWorker.perform_async
14. Refresh the Advanced Search Settings page and verify that Pause Elasticsearch indexing is checked and disabled
15. change the migration completed? to return true
16. Start a Migration Worker Elastic::MigrationWorker.perform_async
17. Refresh the Advanced Search Settings page and verify that Pause Elasticsearch indexing is checked and enabled

Example migration file (does not require pause indexing)

# frozen_string_literal: true

class TestMigration < Elastic::Migration
  batched!
  throttle_delay 10.seconds

  def migrate
    # no op
  end

  def completed?
    false
  end
end

Example migration file (requires pause indexing)

# frozen_string_literal: true

class TestMigration < Elastic::Migration
  batched!
  throttle_delay 10.seconds
  pause_indexing!

  def migrate
    # no op
  end

  def completed?
    false
  end
end

Screenshots (strongly suggested)

This should only show up when a migration requires indexing to be paused

Does this MR meet the acceptance criteria?

Conformity

• I have included changelog trailers, or none are needed. (Does this MR need a changelog?)
• I have added/updated documentation, or it's not needed. (Is documentation required?)
• 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?)
• 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