[Microbatch] Use behavior flag to gate microbatch functionality

[QMalcolm]

In #308 we began gating the microbatch functionality behind an environment variable. However, for a better user experience, we're moving to using a project flag in dbt-core (dbt-labs/dbt-core#10799). In dbt-adapters, project flags can be interpreted by behavior flags. Thus we need to update dbt-adapters to use a behavior flag for gating the microbatch functionality instead of using an environment variable.