The keyboard shortcuts `[` and `]` should change the brush size, but
they didn't for the tools in GPv3 because GPv3 switched to the 'unified
paint settings' system for brushes.
This PR is a fix for that. The brush scale size operator now also acts
on the unified brush size for GPv3 brushes. One exception: the Grease
Pencil brushes in Paint mode (Draw, Erase, Tint), because they don't
use the unified paint settings.
Pull Request: https://projects.blender.org/blender/blender/pulls/124069
dd5f34eb51