…ING (#814)

* fix!: `to_gbq` loads `unit8` columns to BigQuery INT64 instead of STRING

fix!: `to_gbq` loads naive (no timezone) columns to BigQuery DATETIME instead of TIMESTAMP (#814)
fix!: `to_gbq` loads object column containing bool values to BOOLEAN instead of STRING (#814)
fix!: `to_gbq` loads object column containing dictionary values to STRUCT instead of STRING (#814)
deps: min pyarrow is now 4.0.0 to support compliant nested types (#814)
Release-As: 0.24.0

* chore: adds deprecation warnings related to Python 3.7 3.8

* 🦉 Updates from OwlBot post-processor

See https://github.com/googleapis/repo-automation-bots/blob/main/packages/owl-bot/README.md

* Update pandas_gbq/__init__.py

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* adds the capability to include customer user agent string

* Adds deprecation warning

* 🦉 Updates from OwlBot post-processor

See https://github.com/googleapis/repo-automation-bots/blob/main/packages/owl-bot/README.md

* clarifies purpose and asert for two tests regarding old verbose arg

* 🦉 Updates from OwlBot post-processor

See https://github.com/googleapis/repo-automation-bots/blob/main/packages/owl-bot/README.md

* updates comment regarding verbosity

* Update pandas_gbq/gbq.py

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Co-authored-by: release-please[bot] <55107282+release-please[bot]@users.noreply.github.com>