Fix publishing workflow

Author: sylhare

.github/workflows/gem-publish.yml

@@ -17,10 +17,10 @@ jobs:
 
     steps:
     - uses: actions/checkout@v2
-    - name: Set up Ruby 2.6
+    - name: Set up Ruby 3.1
       uses: actions/setup-ruby@v1
       with:
-        ruby-version: 2.6.x
+        ruby-version: 3.1.x
 
     - name: Publish to GPR
       continue-on-error: true