ci: bump Ruby to 3.2, changes core test matrix to 3.x (#1325)

Ruby 3.2 has stabilized (and the bug with Liquid has been patched). I think this is a good time for us to bump all of our versions, especially with the discussion in #1307.

Separately, I've:

- changed the core test matrix to be across `3.0`, `3.1`, and `3.2`. Ruby 2 has been EOL for ~ 4 months, while 3.0 is around for about 7 more months. 
- standardized the formatting of our CI ruby versions to be strings; this is because the number `3.0` gets YAML-casted to `3`

.github/workflows/publish-gem.yml

@@ -10,10 +10,10 @@ jobs:
 
     steps:
     - uses: actions/checkout@v3
-    - name: Set up Ruby 3.1
+    - name: Set up Ruby 3.2
       uses: actions/setup-ruby@v1
       with:
-        ruby-version: 3.1
+        ruby-version: "3.2"
 
     - name: Publish to GPR
       run: |