Swift: bump python version to 3.8 in workflow

Author: redsun82

.github/workflows/swift-codegen.yml

@@ -15,7 +15,7 @@ jobs:
       - uses: actions/checkout@v2
       - uses: actions/setup-python@v3
         with:
-          python-version: '~3.7'
+          python-version: '~3.8'
           cache: 'pip'
       - uses: ./.github/actions/fetch-codeql
       - uses: bazelbuild/setup-bazelisk@v2