ci: request Ruby 4 in cross-gem as semver requirement

Author: kateinoigakukun

.github/workflows/build-gems.yml

@@ -60,7 +60,7 @@ jobs:
         id: cross-gem
         env:
           PLATFORM: ${{ matrix.ruby-platform }}
-          RUBY_VERSIONS: ${{ join(fromJSON(needs.ci-data.outputs.result).stable-ruby-versions, ',') }}
+          RUBY_VERSIONS: ${{ replace(join(fromJSON(needs.ci-data.outputs.result).stable-ruby-versions, ','), '4.0', '~> 4.0.0') }}
           RB_SYS_VERSION: ${{ steps.rb-sys.outputs.version }}
         run: |
           gem install rb_sys -v "$RB_SYS_VERSION"