PR Summary

This pull request updates the regular expressions used in the release notes check script to improve flexibility and case sensitivity. The changes include:

• Modifying the regex pattern to optionally match the word "External" before "Release Notes" so that both "## Release Notes" and "## External Release Notes" headers are processed correctly.
• Adding the IGNORECASE flag to the regex, making the matching process case-insensitive.

These enhancements ensure that the CI process correctly identifies valid release notes regardless of slight variations in the header text, leading to more robust release note verification in pull requests.

Test Suggestions

• Create PR descriptions with both '## Release Notes' and '## External Release Notes' headers to ensure both forms are correctly matched.
• Test with varying cases (e.g., '## external release notes', '## RELEASE NOTES') to verify case insensitivity.
• Include a scenario with missing or placeholder release notes text to ensure the CI check returns false as expected.
• Test with additional markdown sections following the release notes to ensure the regex stops at the right point.