Add Conflicts: docker-buildx-plugin

Closes: #1120500

Dear Maintainers,

when "migrating" from the docker-ce stack to the official Debian stack, dpkg fails with:

Unpacking docker-buildx (0.19.3+ds1-4) ...
dpkg: error processing archive /tmp/apt-dpkg-install-vyX2Kc/07-docker-
buildx_0.19.3+ds1-4_amd64.deb (--unpack):
 trying to overwrite '/usr/libexec/docker/cli-plugins/docker-buildx', which is
also in package docker-buildx-plugin (0.29.1-1~debian.13~trixie)
dpkg-deb: error: paste subprocess was killed by signal (Broken pipe)
...
Errors were encountered while processing:
 /tmp/apt-dpkg-install-vyX2Kc/07-docker-buildx_0.19.3+ds1-4_amd64.deb
needrestart is being skipped since dpkg has failed
Error: Sub-process /usr/bin/dpkg returned an error code (1)

which could be easily prevented by adding a conflict to the package (as is conveniently done for docker.io against docker-ce).

I'm aware that this is just a nice-to-have since docker-ce is not in the official Debian repositories, but I'm gonna send an MR on Salsa to make it easy.

Best regards, Moritz