Fixes #75

We leverage this action in a composite across multiple repos but some repos have a "Do Not Merge" label spelled in different ways. In order to appropriately handle this, we would have to do something like:

- uses: mheap/github-action-required-labels@v5
  with:
    mode: exactly
    count: 0
    labels: "DO NOT MERGE, Do Not Merge, Do not merge, do not merge"
    add_comment: true

This change makes it so you can provide just one of these labels and it will match regardless of casing.

I left it out of the regex matching cause that might get messy, however I could be misunderstanding the logic here. Are people able to pass in their own regex flags? And if not, do you think it would be safe for me to add the i flag in code here? I answered my own question after looking harder at the code and made the regex matching case-insensitive too. Please let me know if I'm wrong here!