For parity with other repos, make readme.md, changelog.md etc file names uppercase

For parity with the other repos, I think it might be best if we renamed the various files in the repo root to use the same case as we use elsewhere (and that much of the rest of the ecosystem uses)? (eg `CHANGELOG.md`, `README.md` etc)

_Originally posted by @edmorley in https://github.com/heroku/heroku-buildpack-nginx/pull/137#discussion_r2142917858_
            