Removing rule severity for a category of analyzer rules + switching some rules from dotnet_diagnostic syntax

[TalZaccai]

So it turns out that severity of specific "dotnet_diagnostic" style rules take precedence over severity of category of analyzer rules BUT severity of category of analyzer rules takes precedence over dotnet_style_ / csharp_style_ rules. More about this here. So in essence, any specified severity in dotnet_style_ / csharp_style_ rules is effectively ignored and marked as "error". I think then the "appropriate" way to do this is to remove the category severity of error, and run "dotnet format --verify-no-changes" in the CI to make sure that current formatting follows the rules. LMK if you have any comments.