[release/9.0] Add more specific messages when pending model changes are detected

[AndriySvyryd]

Fixes #35133
Addresses comments in #34431, #35080 and #35181

Description

EF offers a way of updating the database schema to match the current model via incremental migrations. The common scenario is to add a new migration, then update the database to the latest migration. But it is easy to forget adding a new migration after the model was changed. So, in 9.0 we added a runtime warning that would throw by default when updating the database if there are any changes detected in the model that aren't reflected in the latest migration.

However, there are several reasons why the migrations could be out-of-date and the exception message was not helpful for some of the users. This PR adds more specific messages and makes the warning to be logged instead of throwing for some cases:

1. There are no migrations at all. This is common when the database is updated through other means. The exception will not be thrown.
2. There are some migrations, but the model snapshot is missing. This is common for migrations added manually. The exception will not be thrown.
3. The model wasn't modified by the developer, but it's built in a non-deterministic way causing EF to detect it as modified. This is common when new DateTime() or Guid.NewGuid() are used in seed data. The exception will still be thrown.
4. For other cases the exception will still be thrown.

We will also add documentation explaining the above.

Customer impact

After the upgrade some applications that were working correctly started crashing on startup. This was by-design, but the required action from the developer wasn't clear. The workaround is to ignore the warning using options.

How found

Multiple customer reports on 9.

Regression

Yes, from 8.

Testing

Tests added.

Risk

Low.

[roji]

Looks great - just suggesting a bit more info in the non-deterministic message.