Bump golang in /internal/witness/cmd/witness (#1299)

Bumps golang from 1.21.5-bookworm to 1.21.6-bookworm.

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

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

internal/witness/cmd/witness/Dockerfile

@@ -1,4 +1,4 @@
-FROM golang:1.21.5-bookworm@sha256:2d3b13c2a6368032e9697e64e7c923184e6e3be03cf01eadff27de124114e64e AS builder
+FROM golang:1.21.6-bookworm@sha256:688ad7f282ce711cc28676531f45d5b840790e28a689fbd4e8e4ac181e23e9c4 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:1.21.5-bookworm@sha256:2d3b13c2a6368032e9697e64e7c923184e6e3be03cf01eadff27de124114e64e
+FROM golang:1.21.6-bookworm@sha256:688ad7f282ce711cc28676531f45d5b840790e28a689fbd4e8e4ac181e23e9c4
 
 COPY --from=builder /build/bin/witness /bin/witness
 ENTRYPOINT ["/bin/witness"]