ci: change github ci to use ubuntu 20.04

Signed-off-by: Huaqi Fang <578567190@qq.com>

.github/workflows/build_sdk.yaml

@@ -33,7 +33,7 @@ jobs:
     runs-on: ${{ matrix.os }}
     strategy:
       matrix:
-        os: [ubuntu-18.04]
+        os: [ubuntu-20.04]
         soc: [demosoc, gd32vf103]
 
     steps:

.github/workflows/deploy_doc.yaml

@@ -22,7 +22,7 @@ jobs:
     runs-on: ${{ matrix.os }}
     strategy:
       matrix:
-        os: [ubuntu-18.04]
+        os: [ubuntu-20.04]
 
     steps:
     - uses: actions/checkout@v3