ci: upgrade sdl_ci.yml and native_backend_tests_ci.yml, to ubuntu-20.04, to avoid ci failures, due to the brownout period, for the deprecated ubuntu-18.04

.github/workflows/native_backend_tests_ci.yml

@@ -53,7 +53,7 @@ jobs:
     strategy:
       matrix:
         os: [
-          ubuntu-18.04, ubuntu-20.04,
+          ubuntu-20.04,
           macos-11, macos-12,
           windows-2019, windows-2022
         ]

.github/workflows/sdl_ci.yml

@@ -16,13 +16,13 @@ concurrency:
 
 jobs:
   v-compiles-sdl-examples:
-    runs-on: ubuntu-18.04
+    runs-on: ubuntu-20.04
     if: github.event_name != 'push' || github.event.ref == 'refs/heads/master' || github.event.repository.full_name != 'vlang/v'
     timeout-minutes: 30
     env:
       VFLAGS: -cc tcc
     steps:
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@v3
       - name: Build V
         run: make && sudo ./v symlink