Refactor to use new batch context varaibles

[QMalcolm]

resolves #966

Problem

During the betas/rcs of dbt-core 1.9.0, the varaibles __dbt_internal_event_time_start and __dbt_internal_event_time_end were available on the model in the jinja context. However when the final of dbt-core 1.9.0 was released, dbt-core started providing a batch object on the model available in the jinja context. This batch object made availble all the prevous information and more, and it is the "correct" way to get the information (i.e. __dbt_internal_event_time_end and __dbt_internal_event_time_start are deprecated.

Solution

Update the microbatch incremental strategies to use model.batch.<variable> variables

Checklist

• I have read the contributing guide and understand what's expected of me
• I have run this code in development and it appears to resolve the stated issue
• This PR includes tests, or tests are not required/relevant for this PR
• This PR has no interface changes (e.g. macros, cli, logs, json artifacts, config files, adapter interface, etc) or this PR has already received feedback and approval from Product or DX