[Bug] Make CLA username checks case insensitive

[LakshyaKhatri]

I recently signed a CLA with Open edX. The CLA got accepted successfully, but even after that I was not able to pass the CLA check pipeline in the PR that I opened: openedx/edx-platform#33233 (comment). The issue turned out to be that pipeline is doing a case-sensitive lookup on the usernames (i.e. LakshyaKhatri is not same as lakshyakhatri). I later have to go signup the CLA again (with username as LakshyaKhatri) to make the pipeline pass.

The username lookups in the pipeline should be case-insensitive like it is on GitHub, for example the following usernames will redirect to the same profile:

• https://github.com/LakshyaKhatri
• https://github.com/lakshyakhatri