Update docs to notify customers that NU3043 warning is promoted to an error.

[kartheekp-ms]

Update docs during NuGet 6.13 release timeframe to notify customers that NU3043 warning is promoted to an error.

[kartheekp-ms]

Here are the docs PRs that were merged when the NU3043 warning was added to the product. While addressing this issue, we should consider updating the following docs.

• add docs for new NuGet warning NU3043 docs.microsoft.com-nuget#3339
• deprecate SHA-1 usage in nuget.exe sign command docs.microsoft.com-nuget#3342
• deprecate SHA-1 usage in dotnet nuget sign command dotnet/docs#42697