Administration: Add a `primary` class to the FTP Credentials Modal submit button.

[audrasjb]

Testing steps:

1. Add this to your wp-config.php: define( 'FS_METHOD', 'ftpext' );
2. Install an old version of a plugin
3. Browse to wp-admin/plugins.php
4. Click on the update now link in the plugin’s table row

https://core.trac.wordpress.org/ticket/37647

[github-actions[bot]]

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Core Committers: Use this line as a base for the props when committing in SVN:

Props audrasjb.

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[github-actions[bot]]

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The changes in this pull request can previewed and tested using a WordPress Playground instance.

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Some things to be aware of

• The Plugin and Theme Directories cannot be accessed within Playground.
• All changes will be lost when closing a tab with a Playground instance.
• All changes will be lost when refreshing the page.
• A fresh instance is created each time the link below is clicked.
• Every time this pull request is updated, a new ZIP file containing all changes is created. If changes are not reflected in the Playground instance, it's possible that the most recent build failed, or has not completed. Check the list of workflow runs to be sure.

For more details about these limitations and more, check out the Limitations page in the WordPress Playground documentation.

Test this pull request with WordPress Playground.

[audrasjb]

Committed in https://core.trac.wordpress.org/changeset/58149