Fix Sidekiq crash when gitlab.yml contains UTF-8 characters

What does this MR do and why?

In GitLab 16.0, we migrated away from Settingslogic in !113040 (merged) 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: https://github.com/rails/rails/blob/v6.1.7.2/railties/lib/rails.rb#L21C1-L24
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.

Note that users can work around the issue by setting LANG:

gitlab_rails['env'] = {
  'LANG' => 'C.UTF-8'
}

Relates to #412767 (closed)

How to set up and validate locally

1. In Omnibus GitLab 16.0.1, set /etc/gitlab/gitlab.rb:

gitlab_rails['gitlab_email_display_name'] = 'GitLab SambaÉdu'

2. Run gitlab-ctl reconfigure.
3. Run gitlab-ctl tail sidekiq and watch the errors.
4. Apply this change in /opt/gitlab/embedded/service/gitlab-rails/lib/gitlab_settings/settings.rb.
5. Sidekiq should start.

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