chore(deps): update golang docker tag to v1.20 (#11)

This PR contains the following updates:

| Package | Type | Update | Change |
|---|---|---|---|
| golang | docker | minor | `1.19` -> `1.20` |

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Reviewed-on: #11
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.drone.yml

@@ -5,7 +5,7 @@ name: test
 
 steps:
   - name: test
-    image: golang:1.19
+    image: golang:1.20
     commands:
       - go test -race -coverprofile=coverage.txt -covermode=atomic ./...
 
@@ -23,7 +23,7 @@ name: check-go-mod
 
 steps:
   - name: check go.mod
-    image: golang:1.19
+    image: golang:1.20
     commands:
       - go mod tidy
       - git diff --exit-code go.mod