Change BreadcrumbLevel.Critical to BreadcrumbLevel.Fatal for consistency with other SDKs

[jamescrosswell]

The developer docs indicates:

Allowed values are, from highest to lowest: fatal, error, warning, info, and debug.

Currently the .NET SDK doesn't uses the value critical where other SDKs are using fatal.

This would also be more consistent with the SentryLevel type.

This would obviously be a breaking change, so we'd have to do it in the next major version.