fixes for airflow_pools submodule and update dependencies

[VincentGoeckeritz]

Fixes:

• fix the airflow CLI command (as far as I can determine from the airflow docs even in versions 1.x it was always airflow pools, never pool)
• rename the project_id variable to project (this is what the gcloud command uses and using _id when actually the name is required is misleading)

New:

• add optional variable include_deferred (defaults to false) to allow users to specify whether deferred tasks should be included in the pool's occupied slot calculation
• add a section in the README for how to correctly destroy a pool when using this module

Updates:

• update used the gcloud command terraform module from version ~> 3.1 to ~> 4.0

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