Admin Page: better handle i18n for disconnection warnings

[jeherve]

Fixes #18391

Changes proposed in this Pull Request:

• Let's ensure that we display the right string regardless of the language and the number of connected plugins.

Jetpack product discussion

• N/A

Does this pull request change what data or activity we track or use?

• No

Testing instructions:

• Start from a site connected to WordPress.com and running this branch.
• Add and activate a test plugin like this one:

<?php
/** Plugin Name: Jetpack Connection Test Plugin (1) */
add_action( 'plugins_loaded', function() {
        ( new Automattic\Jetpack\Config() )->ensure(
                'connection',
                array(
                      	'slug' => 'jetpack-connection-test-1',
                        'name' => 'Jetpack Connection Test Plugin (1)'
                )
        );
}, 1 );

• Go to Jetpack > Dashboard, and click on the Manage connection link towards the end of the page.

• You should see this:
  

• Now add and activate a second plugin:

<?php
/** Plugin Name: Jetpack Connection Test Plugin (2) */
add_action( 'plugins_loaded', function() {
        ( new Automattic\Jetpack\Config() )->ensure(
                'connection',
                array(
                      	'slug' => 'jetpack-connection-test-2',
                        'name' => 'Jetpack Connection Test Plugin (2)'
                )
        );
}, 1 );

• You should now see this:
  

Proposed changelog entry for your changes:

• N/A

[jetpackbot]

Scheduled Jetpack release: February 2, 2021.
Scheduled code freeze: January 25, 2021

Thank you for the great PR description!

When this PR is ready for review, please apply the [Status] Needs Review label. If you are an a11n, please have someone from your team review the code if possible. The Jetpack team will also review this PR and merge it to be included in the next Jetpack release.

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