Also removed deprecated ubuntu-18.04

Author: hsbt

.github/workflows/build.yml

@@ -42,7 +42,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        os: [ ubuntu-18.04, ubuntu-20.04, ubuntu-22.04, macos-11 ]
+        os: [ ubuntu-20.04, ubuntu-22.04, macos-11 ]
         name: [ head, debug ]
     runs-on: ${{ matrix.os }}
     steps: