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

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.