Sharing: deprecate wp-admin > Settings > Sharing page

### Impacted plugin

Jetpack

### What

Let's consolidate Sharing / Like settings by deprecating the Settings > Sharing page in wp-admin. There are other places where folks can manage their settings.

### How

It's worth noting that even though the page does not include any Publicize or Verification tools settings, it is still needed for those modules to work, so this would have to be taken into account when deprecating the page:
https://github.com/Automattic/jetpack/blob/12c1ec934857a0c0fa2afe2ddecbddb7cdbf2e4e/projects/plugins/jetpack/modules/publicize/ui.php#L37-L40
https://github.com/Automattic/jetpack/blob/c52e77b1b4151e53a430b2d6ceb0adfb4e468faf/projects/plugins/jetpack/_inc/lib/class.jetpack-keyring-service-helper.php#L56

We will also need to account for options that are currently only available in that screen:

- The option to add custom sharing services.
- The option to use your own custom assets to manage the look of the sharing buttons.


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Previous art on this: #11314

Discussion:

p8oabR-K9-p2/#comment-5540

Start with projects/plugins/jetpack/modules/publicize/ui.php and projects/plugins/jetpack/_inc/lib/class.jetpack-keyring-service-helper.php to understand dependencies on the Sharing settings page. Review previous issue #11314 and the linked discussion, then trace where custom sharing services and custom button assets are stored; done means the page is deprecated without breaking Publicize, Verification, or those options.

Written by the indexing model from the issue text.