Support SQLAlchemy 2.1

[un-def]

SQLAlchemy was capped below 2.1 because sqlalchemy-utils didn't support it (kvesteri/sqlalchemy-utils#800). sqlalchemy-utils 0.43.0 adds SQLAlchemy 2.1 support, so require it and drop the cap. Python 3.10, which SQLAlchemy 2.1 doesn't support, still gets 2.0.

Also:

• delete_metrics() no longer runs its two DELETE statements concurrently with asyncio.gather() in separate sessions. In tests, StaticPool makes both sessions share one SQLite connection. In 2.1, the aiosqlite adapter runs execute + fetchall under a per-connection mutex that commit() doesn't take, so one session's COMMIT could land between the other session's DELETE ... RETURNING and its fetch, and fail with "cannot commit transaction - SQL statements in progress". Under 2.0 the interleaving happened to be safe. Production uses a connection pool, so only tests were affected. Both deletes now run in one session.
• noload() is deprecated in 2.1. Replace it with raiseload(), which also skips the mapped lazy="selectin" load, but raises on access instead of returning an empty collection. RunModel.jobs isn't read after list_projects_run_models(), so the swap is enough there. FleetModel.instances is read downstream for candidate fleets that have no matching instances, so set it to an empty list explicitly with set_committed_value().