Drop Python 3.6 support

[netsettler]

The actual change is pretty simple:

• Drop Python 3.6 support in pyproject.toml
• Adjust GA workflow.
• Update CHANGELOG.rst

NOTE: I think we should hold off on this change for a few days until other things have subsided, since a lot of repos would need to incorporate an incompatible version update (dcicutils = "^5.0.0" instead of dcicutils = "^4.0.0") to get this.

Also, taking this change should get rid of this warning (from testing on GA):

/home/runner/.cache/pypoetry/virtualenvs/dcicutils--i6xsha8-py3.6/lib/python3.6/site-packages/boto3/compat.py:88: 
PythonDeprecationWarning: Boto3 will no longer support Python 3.6 starting May 30, 2022. 
To continue receiving service updates, bug fixes, and security updates please upgrade to Python 3.7 or later. 
More information can be found here: https://aws.amazon.com/blogs/developer/python-support-policy-updates-for-aws-sdks-and-tools/

[coveralls]

Pull Request Test Coverage Report for Build 3020058297

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 71.238%

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Totals
Change from base Build 3011016392:	0.0%
Covered Lines:	5454
Relevant Lines:	7656

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💛 - Coveralls