SSO: consolidate mentions of the SSO feature

We currently are not always consistent when referring to Jetpack's SSO feature in the interface.

> On the Jetpack settings page (mysite.com/wp-admin/admin.php?page=jetpack#/settings), I see “WordPress.com login”.

Let's aim to use "Secure Sign On" everywhere.

Internal reference:
p8oabR-qW-p2#comment-3595

Matching Calypso issue:
https://github.com/Automattic/wp-calypso/issues/38022

Start at the Jetpack settings page entry point, /wp-admin/admin.php?page=jetpack#/settings, and review the matching Calypso issue #38022 for related scope. Identify every interface reference to the SSO feature and update the wording consistently to “Secure Sign On”; done means no remaining interface mentions use the older terminology.

Written by the indexing model from the issue text.