Make rustup a requirement and switch to rust-toolchain.toml (#30056)

This change makes rustup a requirement for building Servo with `./mach`
and switches to the newer `rust-toolchain.toml` format. The goal here is
to make mach builds more similar to non-mach builds.

- The new format allows listing the required components, removing some of
  the complexity from our mach scripts.
- This means we must raise the required version of rustup to 1.23. The
  current version is 1.26.
- We no longer wrap every call to cargo and rustc in "rustup run" calls
  as both cargo and rustc will take care of installing and using all
  necessary components specified in `rust-toolchain.toml` when run
  inside the project directory.

servobuild.example

@@ -4,12 +4,6 @@
 
 # Tool options
 [tools]
-# If use-rustup is set to false, mach will run for example "cargo build"
-# instead of "rustup run --install <toolchain> cargo build"
-# It is then the user’s responsibility to ensure that cargo and especially rustc
-# in $PATH are versions compatible with Servo.
-use-rustup = true
-
 # If rustc-with-gold is true, will try to find and use gold linker with rustc.
 # Defaults to true
 rustc-with-gold = true