Add Ruby 4.0 to CI

Ruby 4.0.0 has been released, so this PR adds Ruby 4.0 to CI.
https://www.ruby-lang.org/en/news/2025/12/25/ruby-4-0-0-released/

To prevent the CI label from showing "CI / Test Ruby 4" instead of
"CI / Test Ruby 4.0", the matrix entries are changed from floats to strings.

.github/workflows/ci.yml

@@ -7,10 +7,11 @@ jobs:
     strategy:
       matrix:
         entry:
-          - { ruby: 3.2, allowed-failure: false }
-          - { ruby: 3.3, allowed-failure: false }
-          - { ruby: 3.4, allowed-failure: false }
-          - { ruby: head, allowed-failure: true }
+          - { ruby: '3.2', allowed-failure: false }
+          - { ruby: '3.3', allowed-failure: false }
+          - { ruby: '3.4', allowed-failure: false }
+          - { ruby: '4.0', allowed-failure: false }
+          - { ruby: 'head', allowed-failure: true }
     name: Test Ruby ${{ matrix.entry.ruby }}
     steps:
       - uses: actions/checkout@v6

.github/workflows/release.yml

@@ -21,7 +21,7 @@ jobs:
         uses: ruby/setup-ruby@v1
         with:
           bundler-cache: true
-          ruby-version: 3.4
+          ruby-version: 4.0
       - uses: rubygems/release-gem@v1
 
   publish_gh_pages: