Add NOT NULL constraint for `remote_mirrors.project_id`

Problem

As part of our Organization Isolation work we are ensuring that all sharding keys are not nullable.

remote_mirrors has a field project_id that has nullable state, however it should be not nullable.

Suggestion

In %17.1 we removed remote mirror records with empty project_id records (see Remove invalid remote mirror records without "p... (!146640 - merged)).

Next step, add a not null constraint and validation for project_id field in remote_mirrors table

See documentation: https://docs.gitlab.com/ee/development/database/not_null_constraints.html#add-a-not-null-constraint-to-an-existing-column