chore: Hardcode Python version to 3.11 in Smithy-Dafny setup action

Author: lucasmcdonald3

.github/actions/install_smithy_dafny_codegen_dependencies/action.yml

@@ -22,7 +22,7 @@ runs:
     - name: Setup Python, black, and docformatter for code formatting
       uses: actions/setup-python@v6
       with:
-        python-version: ${{ matrix.python-version }}
+        python-version: 3.11
         architecture: x64
     - shell: bash
       run: |