update ubuntu runner (#972)

Author: samuraiGH

.github/workflows/main.yml

@@ -20,7 +20,7 @@ jobs:
     - name: Test
       run: dotnet test --no-build -c Debug Mono.Cecil.sln
   linux:
-    runs-on: ubuntu-20.04
+    runs-on: ubuntu-22.04
     steps:
     - uses: actions/checkout@v1
     - name: Build