Update checklist for adding new language version (#79881)

Author: Julien Couvreur

src/Compilers/CSharp/Test/CommandLine/CommandLineTests.cs

@@ -1722,6 +1722,7 @@ public void LanguageVersionAdded_Canary()
             // - [ ] replace all references to C# "Next" (such as `TestOptions.RegularNext` or `LanguageVersionFacts.CSharpNext`) with the new version and fix failing tests
             // - [ ] update _MaxAvailableLangVersion cap (a relevant test should break when new version is introduced)
             // - [ ] update the "UpgradeProject" codefixer
+            // - [ ] test VS insertion and deal with breaking changes. (note: the runtime repo uses "preview" so breaks are resolved sooner)
             //
             // Other repos also need updates:
             // - [ ] email release management to add to the release notes. See csharp-version in release.json in previous example: https://github.com/dotnet/core/pull/9493