Drop ubuntu-20.04 testing.

Author: sunfishcode

.github/workflows/main.yml

@@ -22,14 +22,11 @@ jobs:
       QEMU_BUILD_VERSION: 8.1.0
     strategy:
       matrix:
-        build: [ubuntu, ubuntu-20.04, i686-linux, aarch64-linux, riscv64-linux]
+        build: [ubuntu, i686-linux, aarch64-linux, riscv64-linux]
         include:
           - build: ubuntu
             os: ubuntu-latest
             host_target: x86_64-unknown-linux-gnu
-          - build: ubuntu-20.04
-            os: ubuntu-20.04
-            host_target: x86_64-unknown-linux-gnu
           - build: i686-linux
             os: ubuntu-latest
             target: i686-unknown-linux-gnu