[CI] Make issue labeler more robust

[snprajwal]

The issue labeler handles commit access requests in a case-sensitive manner. This patch expands that to be case-insensitive.

[tstellar]

The regex is designed to match the summary that is automatically filled in for users when they click on the link in the developer guide. I only want users to apply by clicking the link and I don't want to match anything else.

[snprajwal]

I understand the reasoning - it's just that there might be cases where someone goes through the link, discards the issue, and files it later (e.g. waiting for a PR to be merged before submitting the request). I personally found it easier to just type the issue title out instead of navigating the Developer Policy and clicking again, hence the change to retain the same regex but case-insensitive. We can also close this if you are sure you do not want the labeler to handle manually typed requests.