chore: add support for golang 1.20 / drop 1.15 (#275)

Author: nukosuke

.github/workflows/ci.yml

@@ -7,11 +7,11 @@ jobs:
     strategy:
       matrix:
         go-version:
-          - 1.15.x
           - 1.16.x
           - 1.17.x
           - 1.18.x
           - 1.19.x
+          - 1.20.x
     runs-on: ubuntu-latest
     steps:
     - name: Set up Go 1.x