README: Bump Godot client library to 4.0.x

[you-win]

Description

I have updated the Godot client library to work on the latest version of Godot (4.0.2 at time of writing).

Motivation and Context

The current version of Godot mentioned in the README is very old (3.4.x).

How Has This Been Tested?

Tested OS(s): Windows 10

Godot natively supports most major OSs and Linux distros. The client library does not use any OS-specific code. There is a simple demo scene that was used for testing requests.

Types of changes

• Documentation change (a change to documentation pages)

Checklist:

• [x] I have read the Contributing Guidelines.
• [x] All commit messages are properly formatted and commits squashed where appropriate.
• [x] My code is not on master or a release/* branch.
• [x] The code has been tested.
• [x] I have included updates to all appropriate documentation.