Match tags case-insensitively for uncommitted PRs

[lime]

A pull request for an issue I'm following received a comment (microsoft/TypeScript#62323 (comment)) saying the linked issue hasn't been committed. However, the linked issue (microsoft/TypeScript#58561) seems to be committed, and properly marked as such. I suppose help wanted was changed to Help Wanted in the repo at some point in time.

We could of course just fix the case of the one string literal, let me know if that's preferred. It just felt prudent to switch to a case-insensitive comparison, e.g. in case the check gets used in other repos (I noticed microsoft/typescript-go uses the lowercase tag name).

There are other label checks in the codebase that are case sensitive, let me know if you'd like me to touch those as well. I left them as they are for now.

[lime]

@microsoft-github-policy-service agree

[jakebailey]

Seems fine. This repo doesn't do anything with typescript-go, and probably will never given that repo will be archived once we transfer its contents into the main TS repo.