Remove foreign key from terraform_state_versions to ci_builds

What does this MR do and why?

Removes a foreign key from terraform_state_versions to ci_builds, is it will no longer be valid when the ci_* tables are split into a separate database.

Instead, we use an ActiveRecord association with dependent: :nullify.

See #333412 (closed) and #341321 (closed)

Migrations

== 20210920232025 RemoveCiBuildsForeignKeyFromTerraformStateVersions: migrating
-- transaction_open?()
   -> 0.0000s
-- foreign_keys(:terraform_state_versions)
   -> 0.0067s
-- remove_foreign_key(:terraform_state_versions, :ci_builds)
   -> 0.0050s
== 20210920232025 RemoveCiBuildsForeignKeyFromTerraformStateVersions: migrated (0.0163s)

== 20210920232025 RemoveCiBuildsForeignKeyFromTerraformStateVersions: reverting
-- transaction_open?()
   -> 0.0000s
-- foreign_keys(:terraform_state_versions)
   -> 0.0050s
-- transaction_open?()
   -> 0.0000s
-- execute("ALTER TABLE terraform_state_versions\nADD CONSTRAINT fk_04b91e4a9f\nFOREIGN KEY (ci_build_id)\nREFERENCES ci_builds (id)\nON DELETE SET NULL\nNOT VALID;\n")
   -> 0.0041s
-- transaction_open?()
   -> 0.0000s
-- foreign_keys(:terraform_state_versions)
   -> 0.0028s
-- execute("SET statement_timeout TO 0")
   -> 0.0007s
-- execute("ALTER TABLE terraform_state_versions VALIDATE CONSTRAINT fk_04b91e4a9f;")
   -> 0.0060s
-- execute("RESET statement_timeout")
   -> 0.0007s
== 20210920232025 RemoveCiBuildsForeignKeyFromTerraformStateVersions: reverted (0.0687s)

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

MR acceptance checklist

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Related to #341321 (closed)