treewide: bump structmeta dependency

We are currently on v0.1.4 of `structmeta`, so bump to v0.1.6.

The motivation for bumping this transitive dependency (from
`test-strategy`) is due to the following compilation warning
from rustc:

warning: the following packages contain code that will be
rejected by a future version of Rust: structmeta-derive v0.1.4.

Change-Id: I29aa7f4993becd04a7758026537249c753e6ce00
Reviewed-on: https://gerrit.readyset.name/c/readyset/+/7169
Tested-by: Buildkite CI
Reviewed-by: Luke Osborne <luke@readyset.io>

Author: jasobrown-rs