review_prs: Treat GitHub usernames as case insensitive (#2424)

GitHub usernames are actually case insensitive but case preserving (so
`@foo` and `@Foo` refer to the same user, but GitHub will always refer
to them as whatever the user originally created the account as). This PR
normalizes github usernames to lowercase before doing any comparisons or
"is in set" checks.

This avoids the case where a metadata.json lists `foo` as a maintainer,
but GitHub actually stores it as `Foo`, which would cause the reviewer
bot to incorrectly think `foo` hasn't approved the PR when `Foo` has.
(See
https://github.com/bazelbuild/bazel-central-registry/actions/runs/20799054152/job/59739775028
and search for "7072" for an example.)