* The corresponding tests were removed in e16528e.
  They could be re-added but should generate these files dynamically to avoid confusion.

ruby#596 (comment)

* Simpler and more consistent that way.
* Avoids rerunning the post-install hook every time.

* Is this the best error message ever?

* Fixes ruby#618

* Remove the ASan Debugging section for now as it seems mostly redundant.
  Native extensions are automatically compiled with ASan, so there is nothing special to do to use it.

* See ruby#658

… and 12 can no longer be tested

If set up `rubygems: latest` in workflow,
it will cause the error with Ruby 3.0 because the latest rubygems does not support Ruby 3.0.

Ref. https://rubygems.org/gems/rubygems-update

### Error message
```
Updating RubyGems
  /opt/hostedtoolcache/Ruby/3.0.7/x64/bin/gem --version
  3.2.33
  Default RubyGems version is 3.2.33
  Updating RubyGems to latest version
  /opt/hostedtoolcache/Ruby/3.0.7/x64/bin/gem update --system
  ERROR:  Error installing rubygems-update:
  	rubygems-update-3.6.1 requires Ruby version >= 3.1.0. The current ruby version is 3.0.7.220.
  ERROR:  While executing gem ... (NoMethodError)
      undefined method `version' for nil:NilClass
  Updating rubygems-update
  Took   2.86 seconds
Error: The process '/opt/hostedtoolcache/Ruby/3.0.7/x64/bin/gem' failed with exit code 1
```

### How to reproduce
```
        uses: ruby/setup-ruby@v1
        with:
          ruby-version: "3.0"
          rubygems: latest
```