build: Update qemu-docker to v6.21 (#1107)

Co-authored-by: renovate[bot] <29139614+renovate[bot]@users.noreply.github.com>

Dockerfile

@@ -1,7 +1,7 @@
 ARG VERSION_ARG="latest"
 FROM scratch AS build-amd64
 
-COPY --from=qemux/qemu:6.20 / /
+COPY --from=qemux/qemu:6.21 / /
 
 ARG DEBCONF_NOWARNINGS="yes"
 ARG DEBIAN_FRONTEND="noninteractive"