Debug mode is accessible in any gamemode

The bug

In current versions the "Debug Mode" world generation is shown when clicking the "World Type" mode, and allows you to use gamemodes other than Spectator mode in a debug world.

In 1.15.2 the behavior was:

• "Debug Mode" only appeared when you held Shift while clicking the button
• "Debug Mode" automatically changed the gamemode to Spectator and grayed out the button, preventing you from changing it

Steps to reproduce

1. In the "Create New World" screen, go to "More World Options..."
2. Click on the "World Type" button until you get "Debug Mode"
   It appears without you having to hold down Shift
3. Create the world
   You enter the world in Survival mode