Jetpack: swap external links for regular links

If accessibility could be summed up in a single rule, it’d probably be to let users tailor their browsing experience, such as choosing the font size, zoom level, input mode, etc. By default, most browsers and screen readers allow users to open a link in a different tab. When we force the opening in a new tab, we remove that choice from users.

Unless there’s a good reason to keep them external (there sometimes is, like [not losing the progress of a form](https://href.li/?https://blog.pope.tech/2024/01/02/how-to-make-external-links-accessible/)), let's use regular links. To limit the scope of this issue, let's start with the Jetpack core plugin for now (including the Jetpack connection and My Jetpack).

Approved by the Design team: p6TEKc-7DI-p2#comment-11684

Search the Jetpack core plugin for links that force opening in a new tab, including the Jetpack connection and My Jetpack areas named in the issue. Review each external-link case against the stated exceptions, then verify that in-scope links use regular link behavior while intentional exceptions remain unchanged.

Written by the indexing model from the issue text.