Add NOT NULL on ci_pipeline_schedules#project_id

What does this MR do and why?

Add NOT NULL on ci_pipeline_schedules#project_id

Addresses https://gitlab.com/groups/gitlab-com/gl-infra/tenant-scale/-/wikis/Sharding-Priorities#note_2650075699 && #555251

Important context: !148835 (merged)

Epic: &11670

Query on gitlab.com showing there are no rows with project_id IS NULL : https://postgres.ai/console/gitlab/gitlab-production-ci/sessions/41894/commands/128631

References

• !148835 (merged)
• https://gitlab.com/groups/gitlab-com/gl-infra/tenant-scale/-/wikis/Sharding-Priorities#note_2650075699
• &11670

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