Explore new soft memory limit instead of a memory ballast

[julienduchesne]

Soft memory limit was introduced in Go 1.19: https://tip.golang.org/doc/go1.19#mem while a memory ballast was introduced in Tanka v0.22.0: https://tanka.dev/exporting#using-a-memory-ballast

Both have the goal of making the GC work less hard, without completely disabling it and allowing Go's memory to run wild. We should add some benchmarks to make sure the new option works just as well and deprecate the ballast in favor of soft memory limits