Update gif requirement from 0.11.2 to 0.12.0

Updates the requirements on [gif](https://github.com/image-rs/image-gif) to permit the latest version.
- [Release notes](https://github.com/image-rs/image-gif/releases)
- [Changelog](https://github.com/image-rs/image-gif/blob/master/Changes.md)
- [Commits](image-rs/image-gif@v0.11.2...v0.12.0)

---
updated-dependencies:
- dependency-name: gif
  dependency-type: direct:production
...

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

Author: dependabot[bot]

plotters-bitmap/Cargo.toml

@@ -12,7 +12,7 @@ readme = "README.md"
 # See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
 
 [dependencies]
-gif = { version = "0.11.2", optional = true }
+gif = { version = "0.12.0", optional = true }
 
 [dependencies.plotters-backend]
 path = "../plotters-backend"