Improve background migrations CLI

What does this MR do and why?

This MR adds the following rake tasks to the background migrations CLI:

• gitlab:background_migrations:execute[id] - execute migration by id

  $ bundle exec rails gitlab:background_migrations:execute[ci_10]
  Are you sure you want to execute this migration? yes
  Executing background migration `ci_10`...
  Done.

• gitlab:background_migrations:execute_all - execute all unfinished migrations (across all databases)

  $ bundle exec rails gitlab:background_migrations:execute_all
  Are you sure you want to execute all unfinished migrations? yes
  [main] Executing 6 background migrations...
  [main_85]: Start.
  [main_85]: Done.
  [main_86]: Start.
  [main_86]: Done.
  [main_87]: Start.
  [main_87]: Done.
  [main_88]: Start.
  [main_88]: Done.
  [main_89]: Start.
  [main_89]: Done.
  [main_90]: Start.
  [main_90]: Done.
  [ci] Executing 3 background migrations...
  [ci_8]: Start.
  [ci_8]: Done.
  [ci_9]: Start.
  [ci_9]: Done.
  [ci_10]: Start.
  [ci_10]: Done.

References

• Batched background migrations CLI (&16479)

Screenshots or screen recordings

Before	After

How to set up and validate locally

MR acceptance checklist

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