Bump golang from 1.20 to 1.21 in /conformance/tests (#946)

Bumps golang from 1.20 to 1.21.

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

Signed-off-by: dependabot[bot] <support@github.com>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Author: dependabot[bot]

conformance/tests/Dockerfile

@@ -2,7 +2,7 @@
 # this is here so we can grab the latest version of kind and have dependabot keep it up to date
 FROM kindest/node:v1.27.3
 
-FROM golang:1.20
+FROM golang:1.21
 
 WORKDIR /go/src/github.com/nginxinc/nginx-kubernetes-gateway/conformance/tests/