[GHA] Bumped Ubuntu to ubuntu-latest in all workflows (#161)

Author: alongosz

.github/workflows/ci.yaml

@@ -6,7 +6,7 @@ on:
 jobs:
     run-checks:
         name: Run checks
-        runs-on: ubuntu-20.04
+        runs-on: ubuntu-latest
         steps:
             - name: Checkout
               uses: actions/checkout@v2

.github/workflows/flex-cleanup.yaml

@@ -8,7 +8,7 @@ on:
 jobs:
     flex-cleanup:
         name: Cleanup Flex testing endpoint
-        runs-on: ubuntu-20.04
+        runs-on: ubuntu-latest
         steps:
             - name: Checkout
               uses: actions/checkout@v2