build(deps): bump gfx-backend-metal from 0.5.4 to 0.5.6
Bumps [gfx-backend-metal](https://github.com/gfx-rs/gfx) from 0.5.4 to 0.5.6. - [Release notes](https://github.com/gfx-rs/gfx/releases) - [Changelog](https://github.com/gfx-rs/gfx/blob/master/CHANGELOG.md) - [Commits](https://github.com/gfx-rs/gfx/commits) Signed-off-by: dependabot-preview[bot] <support@dependabot.com>
