Update some GDScript files

[dalexeev]

• Replace not X is Y with X is not Y.
  • This is supported since 4.3, so project.godot and *.import files for such projects have also been updated.
• Replace # warning-ignore:X with @warning_ignore("X") and remove unnecessary warning ignores.
• Convert comment keywords such as NOTE and BUG to uppercase (they are highlighted in the editor).

[aaronfranke]

I'm not totally happy with making a cosmetic change that makes the demos incompatible with earlier Godot releases, because it's a barrier to testing the latest demos on older Godot releases. I would personally wait a year or two for is not to have been out for awhile. But we can go forward with it if you prefer that.

The other changes are just good 👍

[dalexeev]

I would personally wait a year or two for is not to have been out for awhile.

We could put this in a separate PR if you prefer. I just thought that the master branch here is the same as in the main repository, and there are separate branches for 4.3 and 4.2. At least that's what the README says.

[Calinou]

Thanks!

Regarding version compatibility, it's fine for master to support only 4.3 and later (new demos can be 4.4-dev only too). We have stable branches for older releases.