Custom mob spawn eggs can be placed in adventure mode now

Partner Rank: 4

Verification builds: 1.16.100.59
Regression builds: 1.16.40

Summary: Spawn eggs of custom mobs can be used in adventure mode now, when previously they weren't able to.

Impact: I am unaware of any content that is broken by this.

Repro Steps:

1. Open any world with custom mobs
2. Grab a vanilla pig spawn egg and a custom mob spawn egg
3. Enter adventure mode
4. Try to use both

Expected Results: Neither can be spawned

Observed Results: Only the custom mob can be spawned

Screenshots/Videos attached: No