Add Support for CBL-Mariner

[rbhanda]

This issue is created to track progress for the work to enable CBL-Mariner support for .NET Core 2.1, 3.1 and .NET 5.

Product and Test updates

• [All teams] Do a compatibility check for the product with the new OS
• [CoreCLR/CoreFX] Runtime / RID Support update
• [Infra] Ensure that the correct Docker images used by teams are built in the dotnet-buildtools-prereqs (if applicable)
• [Infra] Replace docker tags used in builds across repos and branches
• [CLI team] Update CI legs to latest updated Dockerfile
• [All teams] Update test infrastructure to support the new version
• [Acquisition] Add support within nightly repos
• [Acquisition] Deploy to master (GA) during next patch release cycle

Release team:

• Notify .NET Core teams regarding new OS release
• Add feed for new OS release to packages.microsoft.com (if applicable)
• Update release manifest files
• Create new Docker files to test the new OS
• Update the supported OS content on GitHub and marketing sites.

Document updates:

• Add OS prerequisites to the dependencies document
• Update distro install page

[rbhanda]

dotnet/aspnetcore#33388
dotnet/dotnet-buildtools-prereqs-docker#453
dotnet/dotnet-docker#2881

[adegeo]

@rbhanda What should I do for docs on this?

[rbhanda]

Adding @leecow who can help guide for the docs.