Spectator hotbar icon darkening is hardcoded

Discovered while testing MC-269834.

The bug

The darkening applied to unusable options in the Spectator mode hotbar is hardcoded. While the textures can be changed, their darkening is not.

I do not believe this to be a feature request as 24w09a redid the UI background texture, which was previously dirt hardcoded to be darker, but has now been updated to not have darkening applied by the game itself.

How to reproduce

1. Retexture these icons

2. Observe when not in a usable state

Expected results

The darkened versions would use a different texture.

Actual results

They do not.