When a runner gets deleted, jobs that were run with it still have
runner_id. With this commit, they will start to be nullified.

There will be 3 steps;
1. Add a NOT VALID foreign key constraint to the column to ensure
GitLab doesn't create inconsistent records.
2. Add a data migration, to fix or clean up existing records.
3. Validate the whole table by making the foreign key VALID.

This is the 1st step.

Changelog: other