Bump golang in /internal/witness/cmd/witness

Bumps golang from 1.21.2-bookworm to 1.21.3-bookworm.

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

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

internal/witness/cmd/witness/Dockerfile

@@ -1,4 +1,4 @@
-FROM golang:1.21.2-bookworm@sha256:a44d05d5de3474f8135462903bbf74a0fdb761aec455ff557c467339dc0b729b AS builder
+FROM golang:1.21.3-bookworm@sha256:a94b089bc5cabc71da6088eef7493e1d3cb7ffdd0cda3bf85f4aaaed45a71664 AS builder
 
 ARG GOFLAGS=""
 ENV GOFLAGS=$GOFLAGS
@@ -18,7 +18,7 @@ COPY . .
 RUN go build -o /build/bin/witness ./internal/witness/cmd/witness
 
 # Build release image
-FROM golang:1.21.2-bookworm@sha256:a44d05d5de3474f8135462903bbf74a0fdb761aec455ff557c467339dc0b729b
+FROM golang:1.21.3-bookworm@sha256:a94b089bc5cabc71da6088eef7493e1d3cb7ffdd0cda3bf85f4aaaed45a71664
 
 COPY --from=builder /build/bin/witness /bin/witness
 ENTRYPOINT ["/bin/witness"]