Prepare for iOS SPM

[gumaciel]

What feature or improvement would you like to see?

Add the official support for Godot SPM 4.8 after this PR is merged: godotengine/godot#116939

Edit: Merge successful, waiting for stable release

Attends: poingstudios/godot-admob-ios#142

Is your feature request related to a problem? If so, please describe.

A lot of complex code is not needed in the repo for iOS SPM support

Additional context

No response

[gumaciel]

Observation: It's possible to still support old Godot version, there's no need to drop version.

For this, just need to check if the Godot version is lower than 4.8 to apply the current SPM solution, otherwise it will use the 4.8+ native solution.