Gravel doesn't generate below Y=0

In the latest Java snapshot, gravel can now generate on deepslate layer below Y=0, which Bedrock doesn't have currently. This breaks parity.

Step to Reproduce
1. Go to deepslate level below Y=0, preferably in a cave.
2. Find any gravel on that level.

Expected Result
Gravel can generate below Y=0

Observed Result
No gravel generating below Y=0