Bugfix: Downgrade zstd library to fix rare data corruption at max. compression

In restic 0.16.3, 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
0.16.1 and 0.16.2 were not affected.

Restic now uses the previous version of the library used to compress data, the
same version used by restic 0.16.2. 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 are 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.

https://github.com/restic/restic/issues/4677
https://github.com/restic/restic/pull/4679