Rework and simplify update checking logic

[KoBeWi]

Fixes #92593

I have rewritten the update checking logic based on the new format from godotengine/godot-website#858. Unfortunately there was a nasty bug in the previous version that will make users not get notified about beta2 .-.

• The main simplification is that releases array is no longer iterated; instead only the first (newest) release is checked.
• Removed UpdateStatus::DEV, which was effectively unused and made logic more complex.
• Changed [] to get(), so it's safer now (it didn't crash, but it's better to be safe).

Some more testing appreciated. For testing different versions faster, you can replace the code that loads JSON with a custom one that loads a local file: String s = FileAccess::get_file_as_string("somewhere/versions.json");.

[SpockBauru]

Tested the windows artifact, now it doesn't show anything so... It's working?
How to do the json thing?

[KoBeWi]

If nothing appears it means there is no new version available.
You can get the official JSON here: https://godotengine.org/versions.json
However testing custom versions requires compiling a custom build.

[SpockBauru]

Got the branch on my repo (yes, GitHub Desktop works!), compiled, changed VERSION_STATUS to "dev6" in version_generated.gen.h, compiled again and now it seems to work :D
The link correctly opens https://godotengine.org/download/archive/4.3-beta1/

[akien-mga]

Code looks good, I'll try to do some tests to make sure it works for different build names.

[akien-mga]

Did some quick testing by adding 4.3-beta2, 4.3-stable, and 4.4-dev1 to versions.json, and making a local build named 4.3-beta1. Seems to work as expected.

[akien-mga]

Thanks!