.NET 7 feed updates

[mmitche]

Addresses #10203

• Make RC 1 and RC 2 internal channels
• Add internal variants for the 7.0 and 7.0.1xx GA channels
• Update the setup nuget sources scripts to support net7 and simplify them a bit

To double check:

• The right tests are in and and the right validation has happened. Guidance: https://github.com/dotnet/arcade/tree/main/Documentation/Validation

[riarenas]

I like the changes to the scripts

[mmitche]

I like the changes to the scripts

Yep, they are better. I am wondering whether it's time to move away from the feed credential, and instead use the built-in tasks. I wonder if we are paying a performance penalty for the v2 feed format. We originally used it simply because of the flakiness of the feed auth plugin but that probably no longer applies.

[riarenas]

Hmm yeah it's possible we can now just have the scripts add the feeds to the nuget config and make sure the auth task / x-org scripts are added to the templates / custom repo yaml when running in internal.