Update debian to bullseye (#8287)

Several container images use `debian:buster-20210208-slim`. `bullseye`
is now the default version (i.e., referenced by the `latest` tag).

This change updates container images that use debian to reference
`bullseye` instead of `buster`. The date tags have been dropped so that
we pick up the latest patch version on each Linkerd release.

Signed-off-by: Oliver Gould <ver@buoyant.io>

Dockerfile-debug

@@ -1,4 +1,4 @@
-FROM debian:buster-20210208-slim
+FROM debian:bullseye-slim
 RUN apt-get update && apt-get install -y --no-install-recommends \
     curl \
     dnsutils \

Dockerfile-proxy

@@ -39,7 +39,7 @@ COPY --from=fetch /build/proxy-version /usr/lib/linkerd/linkerd2-proxy-version.t
 COPY --from=fetch /build/linkerd2-proxy /usr/lib/linkerd/linkerd2-proxy
 COPY --from=fetch /build/linkerd-await /usr/lib/linkerd/linkerd-await
 COPY --from=golang /out/proxy-identity /usr/lib/linkerd/linkerd2-proxy-identity
-COPY --from=debian:buster-20210208-slim /bin/sleep /bin/sleep
+COPY --from=debian:bullseye-slim /bin/sleep /bin/sleep
 ARG LINKERD_VERSION
 ENV LINKERD_CONTAINER_VERSION_OVERRIDE=${LINKERD_VERSION}
 ENV LINKERD2_PROXY_LOG=warn,linkerd=info

cni-plugin/Dockerfile

@@ -18,7 +18,7 @@ COPY cni-plugin cni-plugin
 ARG TARGETARCH
 RUN CGO_ENABLED=0 GOOS=linux GOARCH=$TARGETARCH go build -o /go/bin/linkerd-cni -v -mod=readonly ./cni-plugin/
 
-FROM debian:buster-20210208-slim
+FROM debian:bullseye-slim
 WORKDIR /linkerd
 RUN apt-get update && apt-get install -y --no-install-recommends \
     iptables \