Please make it buildable against Debian Bullseye / PureOS Byzantium

These are the upcoming releases and we want to be prepared. Build currently fails like

Option buildtype is: plain [default: debugoptimized]
Found ninja-1.10.0 at /usr/bin/ninja
   dh_auto_build -O--builddirectory=_build -O--buildsystem=meson

	cd _build && LC_ALL=C.UTF-8 ninja -j16 -v
[0/56] '/build/squeekboard-1.9.2~1652.gbp571545/cargo_build.sh' src/librs.a --lib
error: failed to select a version for the requirement `bitflags = "= 1.0.4"`
  candidate versions found which didn't match: 1.2.1
  location searched: directory source `/usr/share/cargo/registry` (which is replacing registry `https://github.com/rust-lang/crates.io-index`)
required by package `rs v0.1.0 (/build/squeekboard-1.9.2~1652.gbp571545)`
perhaps a crate was updated and forgotten to be re-vendored?

See https://arm01.puri.sm/job/debs/view/GNOME%20Bullseye/job/deb-squeekboard-bullseye-aarch64/3/console

side note: squeekboard is the last missing package for basic usability on those platforms (we have phosh / phoc / kernel / uboot).