Update: cicd; ubuntu version.

18.04 is deprecated.
actions/runner-images#6002

Author: scharissis

.github/workflows/cicd.yml

@@ -13,7 +13,7 @@ on:
 jobs:
   build:
     name: Build & Test
-    runs-on: ubuntu-18.04
+    runs-on: ubuntu-latest
     steps:
     - name: Set up Go 1.15
       uses: actions/setup-go@v2