Throw on RelationalEventId.MigrationsNotFound by default

[AndriySvyryd]

In EF 9.0 we started throwing for RelationalEventId.PendingModelChangesWarning, but in 9.0.1 we've excluded the case when there are no migrations.

[SamMonoRT]

Does this need any doc updates? Moving back to 10.0 for tracking.

[AndriySvyryd]

@SamMonoRT Yes, if we start throwing on RelationalEventId.MigrationsNotFound we should probably list it in breaking changes