chore(deps): update dependency jeboehm/mailserver-admin to v5.17.21 (#751)

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

Author: renovate[bot]

target/web/Dockerfile

@@ -79,7 +79,7 @@ RUN --mount=type=cache,target=/root/.composer \
 
 FROM composer AS admin-builder
 
-ARG ADMIN_VER=5.17.20 # renovate: depName=jeboehm/mailserver-admin
+ARG ADMIN_VER=5.17.21 # renovate: depName=jeboehm/mailserver-admin
 WORKDIR /opt/admin
 RUN curl -sSLf \
         -o /tmp/admin.tar.gz \