Drop parse_duration for fundu-systemd (#1246) (#1468)

`parse_duration` is no longer maintained upstream; `fundu-system` seems like the best option to provide most of the functionality provided by parse_duration and minimize deps.

Fixes #1246

Dep diffstat: +3 -6

## Caveats
- I've done basically no testing of this PR beyond `cargo test`
- See my comment in #1246 for regression risk; this is a breaking change. Is there a document I should update to make note of this? Should I rebase this for `next-v3` instead?
- In theory CVE-2021-29932 is fixed by this PR as `fundu-systemd` doesn't support exponents, but I've done no verification that this is really the case beyond the info in #1246

Reviewed-on: https://git.deuxfleurs.fr/Deuxfleurs/garage/pulls/1468
Reviewed-by: Alex <lx@deuxfleurs.fr>

src/util/Cargo.toml

@@ -22,6 +22,8 @@ arc-swap.workspace = true
 async-trait.workspace = true
 blake2.workspace = true
 bytesize.workspace = true
+fundu.workspace = true
+fundu-systemd.workspace = true
 thiserror.workspace = true
 hexdump.workspace = true
 xxhash-rust.workspace = true