Bugfix: Update zstd library to fix possible data corruption at max. compression

In restic 0.16.0, backups where the compression level was set to `max` (using
`--compression max`) could in rare and very specific circumstances result in
data corruption due to a bug in the library used for compressing data.

Restic now uses the latest version of the library used to compress data, which
includes a fix for this issue. Please note that the `auto` compression level
(which restic uses by default) was never affected, and even if you used `max`
compression, chances of being affected by this issue were very small.

To check a repository for any corruption, run `restic check --read-data`. This
will download and verify the whole repository and can be used at any time to
completely verify the integrity of a repository. If the `check` command detects
anomalies, follow the suggested steps.

To simplify any needed repository repair and minimize data loss, there is also
a new and experimental `repair packs` command that salvages all valid data from
the affected pack files (see `restic help repair packs` for more information).

https://github.com/restic/restic/issues/4523
https://github.com/restic/restic/pull/4530
