CI: Bump runner to ubuntu-latest.

Author: Gadgetoid

.github/workflows/micropython.yml

@@ -14,7 +14,7 @@ env:
 jobs:
   build:
     name: ${{ matrix.name }} (${{ matrix.board }} ${{ matrix.variant }} ${{ matrix.modules }})
-    runs-on: ubuntu-20.04
+    runs-on: ubuntu-latest
     continue-on-error: true
     strategy:
       matrix: