This reflects the changes made in
rake-compiler/rake-compiler-dock#135.

While this requires packages to update their `RUBY_CC_VERSION` to use
Ruby patches across the board, this is already needed for Ruby 3.3.5.
This change at least makes this consistent.

See
https://github.com/rake-compiler/rake-compiler-dock/releases/tag/v1.8.0.

This is needed by rack-compiler-dock v1.8.0.

`RakeCompilerDock.cross_rubies` supplies the exact patch release
needed. This is available in rack-compiler-dock v1.9.1:

* rake-compiler/rake-compiler-dock#149
* https://github.com/rake-compiler/rake-compiler-dock/releases/tag/v1.9.1

This simplifies the `Dockerfile`, but it does mean we have to keep the
rake-compiler-dock version in the `Gemfile` in sync with all the
images in `docker/Dockerfile.*`.

rake-compiler-dock now makes a clear distinction between GNU and Musl
builds:
https://github.com/rake-compiler/rake-compiler-dock?tab=readme-ov-file#linux-gnu-and-musl-important-details

This change makes it possible to build images using the explicit
aarch64-linux-gnu and x86_64-linux-gnu targets needed by
rake-compiler-dock.

Previously it was possible with rake-compiler-dock to omit
`RUBY_CC_VERSION`, but now it appears that the versions must
be specified to  work. We default to all supported versions.

ghcr.io/rake-compiler/rake-compiler-dock-image:1.9.1-mri-x86-linux is
now based on an Ubuntu 20.04 image, so we can avoid the CentOS-changes
and do what we do with the x86_64-linux Dockerfile.

`matrix.platform.ruby_target` should now be `env.ruby_platform`. These
were missed in #493.

This excludes `x86-linux` and `x86-mingw32`, which were not built
before. `x86-mingw32` is obsolete.

Closes #503