Enable Geo secondary proxy by default for unified URLs

What does this MR do and why?

Enables the secondary proxying by default for all unified URLs as discussed in #325732 (comment 712711610).

As above, also renames the geo_secondary_proxy feature flag to geo_secondary_proxy_separate_urls, which now only applies to separate URLs and is disabled by default.

For unified URLs, this is now enabled by default when both sites have the same URL.

Relates to #325732 (closed)

How to set up and validate locally

1. Checkout this branch and run make gitlab-workhorse-update && gdk restart gitlab-workhorse on the secondary
2. On a GDK + Geo setup, go to the secondary as usual and notice it be a read-only secondary
3. Change the internal URL of the primary to the URL where that is listening in your case (i.e. http://172.16.123.1:3000/)
4. Change the URL of the secondary to be the same as the primary
5. Go to the secondary as before (ignoring the mismatch with the actual URL here, intentionally mentioned in !74329 (comment 731720004) to make testing easier, wait for a few minutes for the cache to clear, notice it proxy requests to the primary now
6. Change the URL back, wait for the proxy to not happen anymore
7. Enable the FF, Feature.enable(:geo_secondary_proxy_separate_urls)
8. Wait a few minutes again and notice proxying happen with different URLs.

MR acceptance checklist

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