lock ubuntu to 20.04, python 3.6 not available for ubuntu 22.04

Author: Michael Maeng

.github/workflows/ci.yml

@@ -15,8 +15,8 @@ jobs:
     runs-on: ${{ matrix.os }}
     strategy:
       matrix:
-        os: [ ubuntu-latest ]
-        python: [ 3.6.15, 3.7, 3.8, 3.9 ]
+        os: [ ubuntu-20.04 ]
+        python: [ 3.6, 3.7, 3.8, 3.9 ]
         node: [ 14 ]
     env:
       SAM_CLI_TELEMETRY: "0"