use runes instead of bytes when splitting into chunks for loki log upload

Created by: burmudar

In https://github.com/sourcegraph/sourcegraph/pull/36472 we split large log entries into smaller pieces.

Currently it operates on byte slices, but @jhchabran pointed out that this can be an issue with UTF-8 chars as we might split on one when using bytes. Hence, use we should use runes!