Bump png from 0.16.7 to 0.16.8

Bumps [png](https://github.com/image-rs/image-png) from 0.16.7 to 0.16.8.
- [Release notes](https://github.com/image-rs/image-png/releases)
- [Changelog](https://github.com/image-rs/image-png/blob/master/CHANGES.md)
- [Commits](https://github.com/image-rs/image-png/commits)

Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

Cargo.lock

@@ -433,9 +433,9 @@ dependencies = [
 
 [[package]]
 name = "png"
-version = "0.16.7"
+version = "0.16.8"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "dfe7f9f1c730833200b134370e1d5098964231af8450bce9b78ee3ab5278b970"
+checksum = "3c3287920cb847dee3de33d301c463fba14dda99db24214ddf93f83d3021f4c6"
 dependencies = [
  "bitflags",
  "crc32fast",