Bump version to 4.0-rc

It's been 3 years since the release of Godot 3.2 and the start of the
major overhaul of the codebase for Godot 4.0.

And the work on that new version had started even 6 months before with
Juan working on the Vulkan renderer and various core changes in a
feature branch. This got merged in #36098 on Feb 11, 2020, oh well, we
3 days early ;)

Close to 15,000 pull requests have been merged for the 4.0 milestone,
which is half of the total amount of PRs in Godot's open source
lifetime. This is our biggest release by far, and it's finally time to
let it loose.

A huge thankyou to all the contributors who were involved over the
years, with contributions of any kind. Now's the final stretch to iron
out the remaining blocking bugs and release 4.0-stable.

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