Reorganize version categories to only include major versions

[svetogam]

This changes version headers to only include major versions and reorganizes items accordingly, as proposed in #343.

Most version information given in parentheses has been removed. But plugins/scripts and modules that support 2 major versions (such as Godot 3 and Godot 4) still make this note in parentheses and italics: (Godot 3 and 4). The idea is that this kind of parenthetical note is only given for information that is exceptional to what the headers imply. It's italicized to make it easy to notice that the Godot 4 section contains items that support Godot 3 too. I think the italics clarify this strongly enough that I did not have to add the explicit clarifying statement "(Sorted by latest compatible major Godot version.)" next to the header, which I previously proposed. This explicit statement is only in CONTRIBUTING.md.

I'm satisfied that this is an improvement, but it now occurs to me that parenthetical version information could be removed entirely with what might be a better system. This would be to, for plugins/scripts and modules only, use the headers "Godot 4 only", "Godot 3 only", and "Godot 3 and 4". I think this would be a further improvement, but it's more unclear and involves a tradeoff. I can modify the PR to do this, or it can be merged without it.

Benefits of this further change:

• It removes ambiguity.
• It cleans up asset descriptions and makes the list less noisy to scan.
• It's self-documenting enough that CONTRIBUTING.md can remove the clarification, making it simpler to contribute.

Downsides:

• It's less flexible to include other information, if that is desired in the future.
• It removes symmetry by not using the same headers in different sections, making the list less pretty.
• People who only care about the newest version will be slightly inconvenienced for having to look through two sections instead of one. (My guess is that this is the majority of people.)

Example

Plugins and scripts

Scripts that let you do new stuff, or enhance Godot functionality.

Godot 4 Only

• Plugin A - asdf.
• Plugin B - asdf.

Godot 3 and 4

• Plugin C - asdf.
• Plugin D - asdf.

Godot 3 Only

• Plugin E - asdf.
• Plugin F - asdf.

[svetogam]

In case I wasn't clear, I'm ready to merge this. I won't do the "further improvement" right now because it's not clear to me that it would really be an improvement.

[Calinou]

Thanks!