Move to requirements.txt; update dependencies; partial update for NIST NVD CPE API (2.0)

[samuel-gamelin]

These changes achieve the following:

• Convert from Poetry to a requirements.txt-based format for simplicity and portability
  • pyproject.toml is pip-compatible and remains for simple tooling configuration and project metadata
• Update to Python 3.12
• Update a number of dependencies
  • libcst is updated to support Python (up to 3.12)
  • pandas and numpy are updated to their latest 1.x version (2.x are not compatible with this project currently)
• README instructions are updated in accordance with the above
• GitHub Actions verification pipeline (build-and-verify.yaml) is updated to accommodate the new project structure
• Partial support for CPE API 2.0 in the vulnerability querier

Of note:
NIST has decommissioned version 1.0 of their CPE API last year. A few initial changes were made in accordance with the transition guide, primarily pertaining to the request for CPE searches. However, the response schema differs vastly from 1.0 and does not work with our current query implementation. This will have to be addressed separately (#133).

[john-breton]

Tests likely won't pass until support for version 2.0 of the CPE API is implemented. For now, we could just lower the test threshold with the understanding that it be raised once the code is ported to handle the newest version of the API.

[samuel-gamelin]

Coverage requirement has been decreased to 36% to account for CPE-related tests. This threshold can be increased once the migration is addressed through #133 and #105.

[john-breton]

All looks good on my end. I appreciate the change to requirements.txt from Poetry.