build(deps): bump golang from 1.20 to 1.21 in /testdata/project-v4

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>

testdata/project-v4/Dockerfile

@@ -1,5 +1,5 @@
 # Build the manager binary
-FROM golang:1.20 as builder
+FROM golang:1.21 as builder
 ARG TARGETOS
 ARG TARGETARCH