Deprecate toolchain installation

[kateinoigakukun]

Motivation

We now have Swift SDK, and it's going to be the default recommended installation way for its standard.
Maintaining two different installation methods costs a lot, and removing toolchain builds and only maintaining SDK builds allows us a much simpler build workflow and saves CI costs.

Outcome

Reducing build variants allows:

• Faster development iterations
• Speeds up upstreaming work
• Having more experimental build variations (e.g. +tail-call, LTO?)

Steps

• Install SDK instead of toolchain in the carton auto installer
• Update setup-swiftwasm to install SDK instead of toolchain
• Rename release tag convention to use the same tag name with the upstream
  • The reason why we included -wasm- part in our tag was just to distinguish toolchain versions of ours and upstream's in swiftenv, so we no longer need to include it. Let's just align with the upstream to make things simpler.
  • Postpone it to keep having control to apply patches upon swiftlang/swift
• Update book.swiftwasm.org
• Remove toolchain build from swiftwasm-build CI
• Remove "Releasing swiftwasm/swiftwasm-docker" step from our release workflow

[kateinoigakukun]

It might be a good time to move the responsibility for the SDK installation from carton to the user. It's now much easier than toolchain installation. Or might be good time to archive carton by moving "bundling" and "test runner" features to JavaScriptKit as they are almost only for JS host platforms.