Add support for Alpine 3.14

[rbhanda]

This issue is created to track progress for the work to enable Alpine 3.14 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: New Linux Distro Version - Alpine 3.14 dotnet-docker#2937
• [Acquisition] Deploy to master (GA) during next patch release cycle

Release team:

• Notify .NET Core teams regarding new OS release
• Update the supported OS content on GitHub and marketing sites.

Document updates:

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

[wfurt]

All the RID work should be done. 3.1 is here.