Update gem to fix CI error for Ruby 2.6

https://github.com/mtsmfm/language_server-protocol-ruby/actions/runs/9179048644/job/25275521882?pr=104

```
Your RubyGems version (3.0.3.1) has a bug that prevents `required_ruby_version` from working for Bundler. Any scripts that use `gem install bundler` will break as soon as Bundler drops support for your Ruby version. Please upgrade RubyGems to avoid future breakage and silence this warning by running `gem update --system 3.2.3`
/home/runner/work/language_server-protocol-ruby/language_server-protocol-ruby/vendor/bundle/ruby/2.6.0/gems/steep-1.3.0/lib/steep.rb:19:in `require': cannot load such file -- parallel/processor_count (LoadError)
```

Author: mtsmfm

.github/workflows/ubuntu-test.yml

@@ -24,6 +24,8 @@ jobs:
           bundler-cache: true
       - run: gem update --system
         if: matrix.ruby != '2.5' && matrix.ruby != '2.6' && matrix.ruby != '2.7'
+      - run: gem update --system 3.2.3
+        if: matrix.ruby == '2.6'
       - run: bundle exec rake
       - run: bundle exec rake
         env:

.github/workflows/windows-test.yml

@@ -24,6 +24,8 @@ jobs:
           bundler-cache: true
       - run: gem update --system
         if: matrix.ruby != '2.5' && matrix.ruby != '2.6' && matrix.ruby != '2.7'
+      - run: gem update --system 3.2.3
+        if: matrix.ruby == '2.6'
       - run: bundle exec rake
       - run: bundle exec rake
         env: