Backup: "Enter credentials" should link to Calypso Green.

Originally part of https://github.com/Automattic/wp-calypso/issues/50225.

As reported in p1HpG7-bcl-p2#comment-44716, the notice that is displayed in WP Admin asking for backup credentials should link to Calypso Green instead of Calypso Blue.

### Context

To quote @folletto the aim of the changes is...

> [i]n short: any flow that starts in WP Admin should end in WP Admin, any flow that starts in Calypso should end in Calypso. If no other way it’s possible, the switch should be explicit.

(see: p1HpG7-bcl-p2#comment-44627).

#### Guiding principles

- From Calypso Blue (.com), a link to WP Admin that:
- has a .com equivalent, should be changed to that.
- has only a WP Admin equivalent, should have an explicit label (maybe even with the external link icon), so the user knows it’s going outside.
- From WP Admin, a link to .com that:
- has a WP Admin or Calypso Green equivalent, should be changed to that.
- has only a .com page, should still link to that, but find a label that says so explicitly (maybe even with the external link icon), so the user knows it’s going outside.

Start by locating the WP Admin backup-credentials notice and its current Calypso Blue link. Confirm the notice links to Calypso Green instead, then verify the rendered link and relevant backup flow.

Written by the indexing model from the issue text.