[Pkg-rust-maintainers] Bug#1013729: rust-zbus: Please upgrade to version 2.2.0

[Peter Michael Green]

There seems to be a group of 4 closely related packages, zbus, 
zbus-macros, zvariant and zvariant-derive which
should probably be upgraded in lockstep.

2.x (3.x for zvariant) is a semver break, so reverse dependencies need 
to be investigated.

rust-ashpd - fixed upstream, upstream fix involves a semver bump but 
there are no rdeps.
rust-libslirp - no upstream fix, no rdeps, but does produce a binary 
crate. Popcon is low, but we did get a bug report from a real user, so 
it's not totally unused.
rust-secret-service - no upstream fix, no rdeps, no binarys
squeekboard - no upstream fix, seems to be used by the "phosh" desktop.

I can't find any upstream changelog, but I did a quick attempt at 
bumping the dependencies in libslirp-helper and came to the conclusion 
it would be a non-trivial porting job.

There is also the problem that squeekboard is currently failing to build 
on ppc64el due to what may or may not
be a rust bug ( https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=1013273 )

The other option would be to introduce seperate packages for the old 
versions of the crates, so that the
main packages could be upgraded to the new semver, we try to avoid such 
duplication if possible
though and I do not intend to introduce packages myself.

I belive hntourne was also showing an interest in zbus, putting him in cc.