Revert "deps: update v8 to 3.20.17.11"

After the upgrade from 3.20.17.7 to 3.20.17.11, we've begun hitting
random assertions in V8 in memory-constrained / GC-heavy situations.

The assertions all seem to be related to heap allocations and garbage
collection but apart from that, they're all over the place.

This reverts commit 970bdccc38fee27e10ff1a3b68f3fea3417788a1.