Declare supported Godot versions using a semver range rather than a minor version

[Calinou]

Asset versions should be able to declare a supported version range, rather than a single minor version. Many assets (such as artwork) can work with almost any Godot version out there, or work with several minor versions at the same time (e.g. 3.1 and 3.2).

php-semver could be investigated for this. We should also provide example version strings in the asset submission form, for those unfamiliar with semver strings.

[Calinou]

This was partially addressed in https://github.com/Calinou/godot-asset-library-laravel/commit/6584d3cc822e893b48c979aafb8fb2c72cc1deb5. Assets can now claim to be compatible with any Godot version or any minor version of a major version (like Godot 3.x).

[bojidar-bg]

Just a note: In the other version of the asset library I made it so that an asset advertising support for e.g. 3.1.6 is assumed to also work with 3.1.4 and 3.2, but not 3.0 or 4.0. (So, patch version is disregarded, minor version should be same or larger, major has to be same.)

[and-rad]

An elegant way to keep versions up to date automatically could be by using the GitHub REST API to fetch releases regularly via cron jobs or git hooks.