Datacenter Picker: Minor UI tweaks and soft-launch #7649928

See Automattic/dotcom-forge#1239

Proposed Changes

1. Uses US West, etc. as the labels, per feedback pdKhl6-15J-p2#comment-1472
2. Only shows the primary data center in ‘Web Server Settings’ when it was chosen, and displays as a text input pdKhl6-15J-p2#comment-1479
3. Removes the feature flag for a soft launch.

Testing Instructions

1. Create a new WordPress.com site with a business plan.
2. Navigate to ‘Hosting Configuration’ and click ‘Activate’.
3. Verify the <EligibilityWarnings> component displays with the Data Center Picker UI.
4. Verify the Data Center Picker UI is usable, accessible, and looks good in the browsers we support.
5. Click ‘Continue’ and wait for the site to go Atomic.
6. Look at the ‘Web Server Settings’ UI and verify it appears as expected.