Don't upgrade RubyGems when testing C gem

As the requirement for newer RubyGems only applies to the precompiled,
native gems, skip updating RubyGems when testing the regular C Ruby
platform gem.

.github/workflows/tests.yml

@@ -116,22 +116,12 @@ jobs:
       fail-fast: false
       matrix:
         ruby: ["2.6", "2.7", "3.0", "3.1", "3.2", "3.3", "3.4", "truffleruby"]
-        include:
-          - { ruby: "2.6", rubygems: "3.4.22" }
-          - { ruby: "2.7", rubygems: "3.4.22" }
-          - { ruby: "3.0", rubygems: "3.5.23" }
-          - { ruby: "3.1", rubygems: "default" }
-          - { ruby: "3.2", rubygems: "default" }
-          - { ruby: "3.3", rubygems: "default" }
-          - { ruby: "3.4", rubygems: "default" }
-          - { ruby: "truffleruby", rubygems: "default" }
     runs-on: "ubuntu-latest"
     steps:
       - uses: actions/checkout@v4
       - uses: ruby/setup-ruby@v1
         with:
           ruby-version: "${{ matrix.ruby }}"
-          rubygems: "${{ matrix.rubygems }}"
           bundler-cache: true
       - uses: actions/download-artifact@v4
         with:
@@ -147,21 +137,12 @@ jobs:
       fail-fast: false
       matrix:
         ruby: ["2.6", "2.7", "3.0", "3.1", "3.2", "3.3", "3.4"]
-        include:
-          - { ruby: "2.6", rubygems: "3.4.22" }
-          - { ruby: "2.7", rubygems: "3.4.22" }
-          - { ruby: "3.0", rubygems: "3.5.23" }
-          - { ruby: "3.1", rubygems: "default" }
-          - { ruby: "3.2", rubygems: "default" }
-          - { ruby: "3.3", rubygems: "default" }
-          - { ruby: "3.4", rubygems: "default" }
     runs-on: "macos-13"
     steps:
       - uses: actions/checkout@v4
       - uses: ruby/setup-ruby@v1
         with:
           ruby-version: "${{ matrix.ruby }}"
-          rubygems: "${{ matrix.rubygems }}"
           bundler-cache: true
       - uses: actions/download-artifact@v4
         with:
@@ -177,17 +158,12 @@ jobs:
       fail-fast: false
       matrix:
         ruby: ["2.6", "2.7", "3.0"]
-        include:
-          - { ruby: "2.6", rubygems: "3.4.22" }
-          - { ruby: "2.7", rubygems: "3.4.22" }
-          - { ruby: "3.0", rubygems: "3.5.23" }
     runs-on: "windows-2019"
     steps:
       - uses: actions/checkout@v4
       - uses: ruby/setup-ruby@v1
         with:
           ruby-version: "${{ matrix.ruby }}"
-          rubygems: "${{ matrix.rubygems }}"
           bundler-cache: true
       - uses: actions/download-artifact@v4
         with: