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
```
When a fixed version is passed, it should be the minimum RubyGems
version for the whole matrix, i.e., RubyGems should not be dowgraded
past the default version of the oldest Ruby in the matrix.
David Rodríguez