fix: require python 3.7+ (#348)

* chore(python): drop python 3.6

Source-Link: googleapis/synthtool@4f89b13
Post-Processor: gcr.io/cloud-devrel-public-resources/owlbot-python:latest@sha256:e7bb19d47c13839fe8c147e50e02e8b6cf5da8edd1af8b82208cd6f66cc2829c

* require python 3.7+ in setup.py

* remove python 3.6 sample configs

* add api_description to .repo-metadata.json

* 🦉 Updates from OwlBot post-processor

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

Co-authored-by: Owl Bot <gcf-owl-bot[bot]@users.noreply.github.com>
Co-authored-by: Anthonios Partheniou <partheniou@google.com>

documentai/snippets/noxfile.py

@@ -88,7 +88,7 @@ def get_pytest_env_vars() -> Dict[str, str]:
 
 # DO NOT EDIT - automatically generated.
 # All versions used to test samples.
-ALL_VERSIONS = ["3.6", "3.7", "3.8", "3.9", "3.10"]
+ALL_VERSIONS = ["3.7", "3.8", "3.9", "3.10"]
 
 # Any default versions that should be ignored.
 IGNORED_VERSIONS = TEST_CONFIG["ignored_versions"]