Add case insensitivity to regex

[NotoriousPyro]

Adding case insensitivity to the regex covers more cases where the repository has named their LICENSES, COPYING and others with non standard names such as Licenses or licenses.

This is to catch cases which go-licenses is unable to such as https://github.com/kr/text/blob/main/License

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[NotoriousPyro]

Code forked to https://github.com/NotoriousPyro/go-licenses/tree/caseinsensitive due to CLA requirement.

[wlynch]

Since the CLA wasn't signed, we can't look at this PR. :(

Since there hasn't been any recent activity, I'm going to close this PR.