chlog: number Ubuntu backports with the per-release SRU scheme

--backport was Debian-only: ~bpo is backports.debian.org's scheme and
its number the Debian release, so Ubuntu targets were rejected outright.
Ubuntu backports have their own documented scheme (Ubuntu version-
strings): the development release's version with a per-release ~YY.MM.1
appended, sorting before it (3.1-1ubuntu2 backported to 22.04 becomes
3.1-1ubuntu2~22.04.1; native 3.1 becomes 3.1~22.04.1) and independent of
the version the target release carries. The .N increments for
subsequent per-release SRU uploads.

backport_series_number becomes backport_suffix_for_series: the release
number comes from the new generic get_series_release_number (version
column of the target series' own distro-info data, leading token kept —
"12" for bookworm, "26.04" out of resolute's "26.04 LTS"; empty column
as on sid/experimental means None), and the suffix is picked per vendor:
~bpoNN+ for Debian (plain integer releases only), ~YY.MM. for Ubuntu.
Unnumbered series still error before anything is written.

Also serialize the changelog tests that mutate the process-global
DEBFULLNAME/DEBEMAIL variables behind a tokio Mutex: run in parallel
they raced each other's identity reads, which started failing
intermittently as generate_entry tests accumulated.

src/main.rs

@@ -176,7 +176,7 @@ fn main() {
             Command::new("chlog")
                 .about("Auto-generate changelog entry, editing it, committing it afterwards")
                 .arg(arg!(-s --series <series> "Target distribution series").required(false))
-                .arg(arg!(--backport "Number the entry as a backport of the target series (1.0-1 becomes 1.0-1~bpo12+1)").required(false)
+                .arg(arg!(--backport "Number the entry as a backport of the target series (Debian: 1.0-1 becomes 1.0-1~bpo12+1; Ubuntu: 3.1-1ubuntu2 becomes 3.1-1ubuntu2~24.04.1)").required(false)
                     .conflicts_with_all(["nmu", "rebuild"]))
                 .arg(arg!(--nmu "Number the entry as a non-maintainer upload (1.0-1 becomes 1.0-1.1, native 1.0 becomes 1.0+nmu1)").required(false)
                     .conflicts_with("rebuild"))