Use FakerPHP Avatar package for user avatar generation in demo data

[armanist]

We need realistic placeholder avatars for users in demo data. Instead of using a custom or unstable package, integrate the stable ottaviano/faker-gravatar
package.

This provider should be used in demo data generation for user avatars while keeping post images using PicsumPhotosProvider.

Proposed Changes:

• Add ottaviano/faker-gravatar as a Composer dependency of the Quantum project.
• Update demo data generation to use this package for user avatars.
• Posts should continue to use PicsumPhotosProvider.
• Ensure the package works seamlessly with Quantum’s demo installer commands.