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 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.