Prepare libgit2 1.11 platform builds

[vik-borisov]

Summary

• update libgit2 build version to 1.9.4
• add iOS plugin packaging and CI build artifacts
• expand Android CI artifacts to all supported ABIs
• remove versioned libgit2 artifact names from desktop packaging

Validation

• YAML parsing checked locally
• podspec Ruby syntax checked locally
• shell syntax for Android action checked locally
• full platform builds not run locally because Flutter/Dart/CMake/Gradle tooling is not available in the local PATH

[vik-borisov]

@Egbertdepauw Hi! I changed some things in the logic. Can you recheck it on the latest version?