ci: use ubuntu-latest instead of ubuntu-20.04

- update the used ubuntu action runner, as `ubuntu-20.04` has been
  officially deprecated, see: actions/runner-images#11101

Author: oleonardolima

.github/workflows/cont_integration.yml

@@ -5,7 +5,7 @@ name: CI
 jobs:
   test:
     name: Test
-    runs-on: ubuntu-20.04
+    runs-on: ubuntu-latest
     env:
       TEST_ELECTRUM_SERVER: electrum.blockstream.info:50001
     strategy: