[DRE-maint] Bug#1075473: fix or update? Re: prometheus-client-mmap FTBFS with gcc 14

[Rebecca N. Palmer]

(Found while looking for semi-random RC bugs to work on, so I may be 
missing context; if so, I apologize.)

Should we be trying to fix this, or does it make more sense to update to 
a newer upstream version?

Upstream have replaced this C extension by a Rust extension:
https://gitlab.com/gitlab-org/ruby/gems/prometheus-client-mmap/-/blob/master/CHANGELOG.md?ref_type=heads

Its only reverse dependency appears to be gitlab, which actually wants 
the newer version:
https://sources.debian.org/src/gitlab/16.8.4-1/Gemfile/?hl=387#L387

We appear to have all the Rust version's dependencies, but rb-sys-env 
(0.1 vs 0.2) and thiserror (1.0 vs 2.0) are too old and nix (0.29 vs 
0.25) is too new.