feat: Add lifecycle ignore of engine_version to primary RDS cluster

[rdelgaizo]

what

Added in lifecycle ignore for engine_version to primary RDS cluster instance resource.

why

When using the module to create an Aurora serverless cluster we were seeing engine_version discrepancy after the RDS cluster had an auto minor version update applied by AWS. This update will prevent Terraform from attempting to roll back the engine version or in some cases of looking up the current AWS engine version try to update it before the maintenance period updates the version.

references