In GitLab 16.0, we migrated away from `Settingslogic` in
https://gitlab.com/gitlab-org/gitlab/-/merge_requests/113040 and
started using `ActiveSupport::ConfigurationFile.parse`.

However, if GitLab Omnibus were configured with non-ASCII characters,
such as German Umlauts, Sidekiq would fail to start up due to:

```
incompatible character encodings: US-ASCII and UTF-8
```

This happened because:

1. Ruby sets the default encoding based on the `LANG` environment.
2. When Omnibus GitLab starts Sidekiq, `LANG` is blank. As a result,
   Ruby's `Encoding.default_external` returns `US-ASCII`.
3. When `ActiveSupport::ConfigurationFile.parse` runs, it attempts to
   scan for certain UTF-8 characters, but this fails in GitLab 16.0
   since the file is read as `US-ASCII`.
4. Just by calling `require 'rails'`, Rails sets the encoding to
    UTF-8.
5. However, `sidekiq-cluster` doesn't attempt to `require 'rails'`
before it attempts to load the settings, and so it will crash if
non-ASCII characters are present.

To fix this, set the encodings inside the `GitlabSettings::Settings`
class to ensure configuration files are parsed as UTF-8.

Relates to https://gitlab.com/gitlab-org/gitlab/-/issues/412767

Changelog: fixed