Users: Show the proposed address in the admin email change confirmation. - #13957

When the site admin email address is changed, the confirmation email only mentions the proposed new address in its last line: "This email has been sent to ###EMAIL###". Nothing tells the recipient that this is the address being requested. That matters most when the change wasn't expected, because the recipient can't tell what they would be approving.

This PR states the new address explicitly and follows the approach suggested in comment 13 for keeping existing customizations working.

• Site admin email (update_option_new_admin_email()): adds "The new administration email address will be: ###NEW_EMAIL###" and removes the ambiguous "This email has been sent to" line.
• Network admin email (update_network_option_new_admin_email()): sends the same kind of email with the same ambiguous line, so it gets the same fix ("...changed to: ###NEW_EMAIL###"). Changing both strings in one release means translators only update this text once, as noted in comment 14.
• New ###NEW_EMAIL### placeholder for the new_admin_email_content and new_network_admin_email_content filters.
• Backward compatibility: ###EMAIL### is documented as deprecated. Right after each filter runs, it is converted to ###NEW_EMAIL###, so filtered email content that still uses ###EMAIL### keeps working unchanged.

Testing Instructions

Automated:

npm run test:php -- --filter UpdateOptionNewAdminEmail
npm run test:php -- -c tests/phpunit/multisite.xml --filter UpdateNetworkOptionNewAdminEmail

The tests cover the default email content (it includes the new address and leaves no unreplaced placeholders) and filtered content using ###NEW_EMAIL### or the deprecated ###EMAIL###, for both the site and the network email.

Manual:

1. Go to Settings > General, change "Administration Email Address" and save.
2. Check the confirmation email sent to the new address, for example with a mail-logging plugin. It now says "The new administration email address will be:" followed by the new address.
3. Add a filter that returns custom text containing ###EMAIL###:
   add_filter( 'new_admin_email_content', fn() => 'Confirm ###EMAIL###' );
   Repeat step 1. The email reads "Confirm ".
4. On multisite, repeat steps 1–2 from Network Admin > Settings with "Network Admin Email".

Trac ticket: https://core.trac.wordpress.org/ticket/43706