Prepare Flintrock 2.1.0 release

[nchammas]

This PR makes the following changes:

• Tweak the license file so GitHub recognizes it.
• Fix a mistake in the manifest file so the change log is included as intended.
• Update the default Amazon Linux 2 AMI.
• Update and trim the main README a bit.
• Adopt pyproject.toml. It is "strongly recommended" and commands like python setup.py sdist bdist_wheel are deprecated in favor of python -m build.
• Trim outdated comments and pin of cryptography from setup.py.
• Update testing code for setting up private VPC.

I tested this PR by manually packaging, installing, and testing Flintrock in an isolated environment:

python -m venv venv
source venv/bin/activate
pip install https://github.com/nchammas/flintrock/archive/release-prep.zip
flintrock --help
flintrock launch ...

Related to #359.