Document project setting changes that only affect new projects in 4.8 upgrade guide

[Calinou]

For projects created on 4.7 or prior that wish to use the same settings as newly created projects, these should be mentioned in the 4.8 upgrade guide:

• Ignore joypads on unfocused window by default for new projects godot#120399
• Implement multi-bounce ambient occlusion approximation godot#115426
• Add new default algorithm of automatic focus strategy for Control nodes godot#120631