Bump png from 0.16.6 to 0.16.8

Bumps [png](https://github.com/image-rs/image-png) from 0.16.6 to 0.16.8.
- [Release notes](https://github.com/image-rs/image-png/releases)
- [Changelog](https://github.com/image-rs/image-png/blob/v0.16.8/CHANGES.md)
- [Commits](image-rs/image-png@v0.16.6...v0.16.8)

---
updated-dependencies:
- dependency-name: png
  dependency-type: indirect
  update-type: version-update:semver-patch
...

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]