CI: update ubuntu-20.04 jobs to ubuntu-latest

Author: newAM

.github/workflows/cron.yml

@@ -27,7 +27,7 @@ jobs:
           GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
 
   rt-ci-linux:
-    runs-on: ubuntu-20.04
+    runs-on: ubuntu-latest
     continue-on-error: ${{ matrix.experimental || false }}
     defaults:
       run:

.github/workflows/rt-ci.yml

@@ -10,7 +10,7 @@ name: cortex-m-rt CI
 
 jobs:
   rt-ci-linux:
-    runs-on: ubuntu-20.04
+    runs-on: ubuntu-latest
     continue-on-error: ${{ matrix.experimental || false }}
     strategy:
       matrix: