refactor(pytest): move EIP report generation from standalone script to `spec_version_checker` pytest plugin

[danceratopz]

The script .github/scripts/generate_eip_report.py introduced in ethereum/execution-spec-tests#1537 parses the pytest console output from check_eip_versions and generates a markdown report that can be included in a github issue. This is a brittle approach; a better way would be to simply write the report from the spec_version_checker pytest plugin itself. Then the code is in the same place and can be more easily tested.

[github-actions]

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