fix(db): make migration statement conditional

[roblambell]

Looks like a migration was realigning after some incident, but we do not have a foreign key constraint it's removing.

This PR makes the statement conditional.

When upgrading from v20241011 to latest:

Before

# dispatch database upgrade
[..]
INFO:Running upgrade 3edb0476365a -> 928b725d64f6, Fixes automatic generation issues:/usr/local/lib/python3.11/site-packages/alembic/runtime/migration.py:run_migrations:618
Traceback (most recent call last):
[..]
  File "/usr/local/lib/python3.11/site-packages/dispatch/database/revisions/tenant/versions/2024-11-04_928b725d64f6.py", line 55, in upgrade
    op.drop_constraint("project_stable_priority_id_fkey", "project", type_="foreignkey")
[..]
sqlalchemy.exc.ProgrammingError: (psycopg2.errors.UndefinedObject) constraint "project_stable_priority_id_fkey" of relation "project" does not exist

[SQL: ALTER TABLE project DROP CONSTRAINT project_stable_priority_id_fkey]
(Background on this error at: http://sqlalche.me/e/13/f405)

After

# dispatch database upgrade
[..]
INFO:Running upgrade 3edb0476365a -> 928b725d64f6, Fixes automatic generation issues:/usr/local/lib/python3.11/site-packages/alembic/runtime/migration.py:run_migrations:618
DEBUG:update 3edb0476365a to 928b725d64f6:/usr/local/lib/python3.11/site-packages/alembic/runtime/migration.py:update_to_step:861
INFO:Running upgrade 928b725d64f6 -> 575ca7d954a8, Adds incident summary to the incident table.:/usr/local/lib/python3.11/site-packages/alembic/runtime/migration.py:run_migrations:618
DEBUG:update 928b725d64f6 to 575ca7d954a8:/usr/local/lib/python3.11/site-packages/alembic/runtime/migration.py:update_to_step:861
INFO:Running upgrade 575ca7d954a8 -> 2d9e4d392ea4, Adding display name to the projct model:/usr/local/lib/python3.11/site-packages/alembic/runtime/migration.py:run_migrations:618
DEBUG:update 575ca7d954a8 to 2d9e4d392ea4:/usr/local/lib/python3.11/site-packages/alembic/runtime/migration.py:update_to_step:861
Success.