doc: update GitHub Actions checkout action v5

Following rubygems/release-gem@9fc8c0f

Author: nicolasleger

trusted-publishing/releasing-gems.md

@@ -24,7 +24,7 @@ jobs:
 
     steps:
       # Set up
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v5
         with:
           persist-credentials: false
       - name: Set up Ruby