ci.yaml: Update os and python matrix

This prevents trailing zeroes from being removed by making the values strings not numbers.

Signed-off-by: Rob Woolley <rob.woolley@windriver.com>

.github/workflows/ci.yaml

@@ -10,8 +10,8 @@ jobs:
     build:
       strategy:
         matrix:
-          os: [ubuntu-18.04, ubuntu-20.04]
-          python: [3.6, 3.7, 3.8, 3.9]
+          os: ["ubuntu-18.04", "ubuntu-20.04", "ubuntu-22.04"]
+          python: ["3.7", "3.8", "3.9", "3.10", "3.11", "3.12"]
       name: superflore tests
       runs-on: ${{matrix.os}}
       steps: