Allow Projects with Freeze Periods to be Deleted

What does this MR do?

The original creation of the foreign key from freeze periods to projects did not set any on delete behaviour.

The migration here deletes and recreates that foreign key with a cascading on delete, allowing the project delete to also delete any linked freeze periods.

Changelog: fixed

migration

== 20210617161348 CascadeDeleteFreezePeriods: migrating =======================
-- transaction_open?()
   -> 0.0000s
-- foreign_keys(:ci_freeze_periods)
   -> 0.0057s
-- execute("ALTER TABLE ci_freeze_periods\nADD CONSTRAINT fk_2e02bbd1a6\nFOREIGN KEY (project_id)\nREFERENCES projects (id)\nON DELETE CASCADE\nNOT VALID;\n")
   -> 0.0028s
-- execute("SET statement_timeout TO 0")
   -> 0.0005s
-- execute("ALTER TABLE ci_freeze_periods VALIDATE CONSTRAINT fk_2e02bbd1a6;")
   -> 0.0075s
-- execute("RESET ALL")
   -> 0.0006s
-- foreign_keys(:ci_freeze_periods)
   -> 0.0021s
-- remove_foreign_key(:ci_freeze_periods, :projects, {:column=>:project_id, :name=>"fk_rails_2e02bbd1a6"})
   -> 0.0035s
== 20210617161348 CascadeDeleteFreezePeriods: migrated (0.0331s) ==============

rollback

== 20210617161348 CascadeDeleteFreezePeriods: reverting =======================
-- transaction_open?()
   -> 0.0000s
-- foreign_keys(:ci_freeze_periods)
   -> 0.0034s
-- execute("ALTER TABLE ci_freeze_periods\nADD CONSTRAINT fk_rails_2e02bbd1a6\nFOREIGN KEY (project_id)\nREFERENCES projects (id)\n\nNOT VALID;\n")
   -> 0.0036s
-- execute("SET statement_timeout TO 0")
   -> 0.0005s
-- execute("ALTER TABLE ci_freeze_periods VALIDATE CONSTRAINT fk_rails_2e02bbd1a6;")
   -> 0.0066s
-- execute("RESET ALL")
   -> 0.0007s
-- foreign_keys(:ci_freeze_periods)
   -> 0.0020s
-- remove_foreign_key(:ci_freeze_periods, :projects, {:column=>:project_id, :name=>"fk_2e02bbd1a6"})
   -> 0.0035s
== 20210617161348 CascadeDeleteFreezePeriods: reverted (0.0286s) ==============

Does this MR meet the acceptance criteria?

Conformity

• I have included changelog trailers, or none are needed. (Does this MR need a changelog?)
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• 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.)
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• I have informed the Infrastructure department of a default or new setting change per definition of done, or it's not needed.

For #329162 (closed)