Bump golang in /internal/witness/cmd/feeder

Bumps golang from 1.19-alpine to 1.21-alpine.

---
updated-dependencies:
- dependency-name: golang
  dependency-type: direct:production
...

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

internal/witness/cmd/feeder/Dockerfile

@@ -1,4 +1,4 @@
-FROM golang:1.19-alpine@sha256:470c8d0638c5b7007a6118baee531c30e0516a18e45b35bff1f8ab92cf8f896d AS builder
+FROM golang:1.21-alpine@sha256:445f34008a77b0b98bf1821bf7ef5e37bb63cc42d22ee7c21cc17041070d134f AS builder
 
 ARG GOFLAGS=""
 ENV GOFLAGS=$GOFLAGS