The signature of this callback was updated to include `returning` in
[ecto 3.11](elixir-ecto/ecto#4277).

The ecto dependency was also bumped to 3.11 to eliminate warning
messages with earlier versions.

This enables upserts using Ecto's `:on_conflict` option on inserts.

There a couple limitations since this isn't backed by a sql database:

  * Ecto queries aren't supported for resolving conflicts.
  * The `:conflict_target` must be the table's primary key.

Additionally, this does not add support for `insert_all`.