Check for operating repos doesn't normalize to lowercase

I just installed this for a repo with an organization whose name contains mixed case. I did not think to mix the case when I provided the `GITHUB_REPO_NAME` environment variable. This cased a 403 error with an error of `{"message":"Not configured for this repository"}`. I updated the organization's name in the environment variable to match the case as found in the payload and then the webhook succeeded.

Recommend normalizing both `repoSplit[0]` and `checkSplit[0]` to lowercase for comparison.

https://github.com/helm/pull-sizer/blob/bb557b11dfb698c211d6a9dd70935c93a97642ee/main.go#L176

Start in main.go around line 176, where the repository names are compared, and review the linked code path for the webhook check. Verify that mixed-case organization names work even when GITHUB_REPO_NAME uses different casing; done means the repository is accepted without the 403 error.

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