Bump version to 4.0-beta \o/

At long last!

We're now pretty much feature complete (with a few exceptions which might
still make their way to future betas) and ready to move on to the consolidation
and bugfixing beta phase.

The API is more or less settled for 4.0, though we still reserve the right to
do changes if we see that it's really for the better - but from here on we'll
do our best to ensure that early adopters of 4.0 beta have an easy upgrade path
to future beta and eventually stable releases.

A million of thanks to all contributors who worked so hard on shaping Godot 4.0
over the past 3 years! Git counts nearly a thousand of you authoring close to
11,000 unique commits (excluding merge commits). You're all breathtaking! <3

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