libsixel: use libpng version range, bump deps

[mayeut]

Specify library name and version: libsixel/all

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• I've read the contributing guidelines.
• I've used a recent Conan client version close to the currently deployed.
• I've tried at least one configuration locally with the conan-center hook activated.

[conan-center-bot]

🚦 Thank for your Bump dependencies PR. The build service will be triggered soon by a Conan team member.

[AbrilRBS]

Thanks!