manage_links should be a custom capability not a default

[Beee]

I just noticed the capability manage_links still gets added to the default administrator capabilities upon a clean install.

If you check that with current_user_can it returns as false, even when a user has this capability, like an administrator. This is when the plugin link manager is not installed.

If you have the plugin link manager installed, it returns as true (as expected).

So I think it should be removed out of the core and inserted into the ​https://wordpress.org/plugins/link-manager/.

[swissspidy]

Related: #42131, #40088.