The `post_effect` debug overlay entry does not upgrade to `post_effects`

When updating from versions before 26.3 Snapshot 3, the post_effects debug overlay entry is reset to “OFF,” no matter what the old post_effect entry was set to.

How to reproduce:

1. Launch 26.3 Snapshot 2 or older.

2. Open the Debug Options menu (F3+F6 by default).

3. Set the post_effect entry to anything other than “OFF.”

4. Close the game, and in the same folder, launch 26.3 Snapshot 3 or newer.

5. Open the Debug Options menu again.
   -> ❌ The post_effects entry is set to “OFF.”

Expected result:

The post_effect debug overlay entry would upgrade to post_effects when updating.

Observed result:

The post_effects debug overlay entry is reset to “OFF” after updating, no matter what the post_effect entry was previously set to.