chore(deps): update golang docker tag to v1.21.5 (#29)

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This PR contains the following updates:

| Package | Type | Update | Change |
|---|---|---|---|
| golang | stage | patch | `1.21.4` -> `1.21.5` |

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cloudevents-server/Dockerfile

@@ -1,4 +1,4 @@
-FROM golang:1.21.4 AS builder
+FROM golang:1.21.5 AS builder
 COPY . /app
 RUN cd /app && go build -o server