Remove .net 6 runtime requirement from dotnet-helix-service

[chcosta]

Dotnet-helix-service has a requirement on the .NET 6 runtime because it utilizes secret-manager, which is not yet updated to .NET 8 support. AFTER #1430 is completed and secret-manager targets .NET 8, we should remove these additional runtime installs from the dotnet-helix-service build.

Sample .NET 6 runtime install - https://dev.azure.com/dnceng/internal/_git/dotnet-helix-service?path=/eng/deploy.yaml&version=GBmain&line=30&lineEnd=34&lineStartColumn=1&lineEndColumn=23&lineStyle=plain&_a=contents

There are more locations in the repo which also install 6.0 and should be updated.

[riarenas]

This issue depends on an issue that is not part of the spike: #2413

@markwilkie @ilyas1974 Should #2413 be moved to the spike, or this issue removed? (With the understanding that this would add at least 2 or 3 weeks to the spike)?

[ilyas1974]

I think this can be removed from the spike (Mark can confirm). Our services will not stop working, we just won't be getting updating.

[riarenas]

Agreed. I can't find anything that would break underneath us, other than getting more CG alerts. Removing from the Spike.