Migrate LGTM.com installation from OAuth to GitHub App

[LGTM Bot]

Hi There,

This project is still using an old implementation of LGTM's automated code review, which has now been disabled. To continue using automated code review, and receive checks on your Pull Requests, please install the GitHub App on this repository.

Thanks,
The LGTM Team

[Lee Rhodes]

This was done by Apache Infrastructure Team