Geo: Turn read-only banner to warning

What does this MR do and why?

Related to #344039 (closed)

• update default message: This GitLab instance is undergoing maintenance and is operating in read-only mode. GitLab is undergoing maintenance
• use Pajamas::AlertComponent instead of old .flash-notice
• split read_only_message logic in 3 parts: read_only_title, read_only_message and geo_secondary_read_only_message

Primary with Maintenance mode

Before	After

Secondary with Maintenance mode

Before	After

Secondary without Maintenance mode

Before	After

How to make this alert appear

1. GitLab Premium In the Admin Area go to Settings > General > Maintenance Mode
2. Enable maintenance mode and add a message, click save changes
3. Refresh page

Admin Area	Banner message