Bump golang in /examples/splunk-hec-traces/tracing

Bumps golang from 1.20.7-bullseye to 1.21.0-bullseye.

---
updated-dependencies:
- dependency-name: golang
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <support@github.com>

examples/splunk-hec-traces/tracing/Dockerfile

@@ -1,4 +1,4 @@
-FROM golang:1.20.7-bullseye
+FROM golang:1.21.0-bullseye
 
 WORKDIR /go/src/app