remove deprecated Actions runner version

Author: djdefi

.github/workflows/main.yml

@@ -6,7 +6,7 @@ jobs:
   build:
     strategy:
       matrix:
-        os: ['ubuntu-22.04', 'ubuntu-20.04', 'ubuntu-18.04', 'macos-latest']
+        os: ['ubuntu-22.04', 'ubuntu-20.04', 'macos-latest']
       fail-fast: false
     runs-on: ${{ matrix.os }}
     steps: