更新 linux sdk ubuntu-20.04，18.04 github已经unsupported

Author: SundoggyNew

.github/workflows/libxp2p_linux.yml

@@ -8,7 +8,7 @@ on:
 
 jobs:
   build-and-deploy:
-    runs-on: ubuntu-18.04
+    runs-on: ubuntu-20.04
     steps:
     - name: Checkout
       uses: actions/checkout@v2