Make Scope.Environment non nullable (next major)

[jamescrosswell]

As of #5365, Scope.Environment can't be null:

sentry-dotnet/src/Sentry/Scope.cs

Lines 158 to 162 in b7dd7fb

	if (value is null)
	{
	Options.LogDebug("Environment cannot be null. Reverting to default value from the options.");
	field = Options.Environment;
	}

We may as well reflect that in the property declaration (which is currently string?).

Note

Breaking change so will need to wait till next major

[linear-code]

NET-741