Relax Ecto Version Constraint

[heywhy]

The current version constraint (3.8.3) is limiting because one can't take advantage of new ecto features because you can't update the installed version (even though new versions of ecto have no changes that affect Etso). So, changing from "~> 3.8.3" to "~> 3.8" fixes this problem. Please treat this as important.

References:

https://github.com/evadne/etso/blob/cae311eedf42003ddbc6796798a67193d5e56454/mix.exs#L30