Fixed default fallback for storage engine for mongo versions > 6.

[davidwaroquiers]

Hello!

Thanks a lot for this nice package!

I stumbled across an issue with mongo versions > 6 (actually already raised in #97). The reason is that the "ephemeralForTest" storage engine has been removed in mongo version 7 onwards.

This pull requests solves this by having a different fallback for mongo versions up to 6 ("ephemeralForTest") and mongo versions larger than 6 ("wiredTiger").