Remove support for .NET 6.0

[MichaelSimons]

.NET 6.0 reached EOL on Nov 12, 2024. Per the Tag Lifecycle documentation, the 6.0 images will continue to be rebuilt upon base image updates for one month. After that time, all 6.0 infra, Dockerfiles, tests, documentation, etc. should be removed. Work can begin in nightly now.

[mthalman]

I was just going to log this. 😄

[lbussell]

Checklist:

• Remove all entries for the EOL .NET version in manifest.json, manifest.versions.json, ImageVersion.cs, TestData.cs, MCR tags metadata templates, and delete the Dockerfiles
• Search for and simplify conditions including the EOL .NET Version in the Dockerfile templates and tests
• Update the path variable appropriately for the dotnet-docker-nightly-pr-no-cache pipeline so that it continues to target active Dockerfiles.
• Replace all references to the EOL .NET version in documentation with a new .NET version
• Remove the EOL .NET version from the .NET Framework SDK images
  • https://github.com/microsoft/dotnet-framework-docker/blob/6a3c9d048f75c1e69c0e1059564cd56a90bf9e3c/eng/dockerfile-templates/sdk/Dockerfile#L56-L59
  • https://github.com/microsoft/dotnet-framework-docker/blob/58a18cf958883c6a67ae888854bd9bb09f085e83/eng/dockerfile-templates/sdk/Dockerfile.ltsc2016#L80-L81
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