ci: Use GITHUB_OUTPUT envvar instead of set-output command (#553)

* ci: Use GITHUB_OUTPUT envvar instead of set-output command

* Quote envvar to match documentation

.github/workflows/ci.yml

@@ -28,7 +28,7 @@ jobs:
       - name: Get full Python version
         id: full-python-version
         shell: bash
-        run: echo ::set-output name=version::$(python -c "import sys; print('-'.join(str(v) for v in sys.version_info))")
+        run: echo "version=$(python -c "import sys; print('-'.join(str(v) for v in sys.version_info))")" >> "$GITHUB_OUTPUT"
 
       - name: Install poetry
         shell: bash