[Pkg-rust-maintainers] Bug#1060874: rust-which: please upgrade to branch v5

[Peter Green]

> Please upgrade to branch v5.

I think it probablly makes more sense to go straight to version
6. I've uploaded that to experimental, can you prepare an update
to precious.

upstream changelog analysis:

nothing that looks too massive/scary, just msrv/dependency
bumps and some slight changes to errors.

preliminary rdep analysis:

precious:
   Jonas's package, upstream still on version 4

rust-bindgen:
   rust team package, upstream has dropped use of which,
   upstream is semver breaking

rust-clearscreen:
   rust team package, upstream has moved to 6, new
   upstream is semver breaking, but upstream did not
   appear to make any code changes when bumping which

rust-docmatic:
   rust team pacakge, upstream is still on version 2

rust-protoc:
   rust team package, upstream is still on version 4

rust-test-with:
   new upstream depends on version 5, new upstream
   is semver-breaking. Upstream did not appear to
   make any code changes when bumping dep.

rust-tree-sitter-cli:
    rust team package, new upstream has removed the
    dependency on which new upstream is semver-breaking

sccache:
    upstream uses version 6, debian is currently
    relaxing