Submission Review Guidelines:

• I have followed the WooCommerce Contributing Guidelines and the WordPress Coding Standards.
• I have checked to ensure there aren't other open Pull Requests for the same update/change.
• I have reviewed my code for security best practices.
• Following the above guidelines will result in quick merges and clear and detailed feedback when appropriate.

Changes proposed in this Pull Request:

Closes #39103.

In Marketing page, there are some button text that says "Install plugin". In the same page, there is another card with the title "Installed extensions". The term "plugin" and "extension" are not consistent.

In this PR, we make them consistent by renaming the button text to "Install extension".

In "Channels" card:

In "Create a new campaign" modal:

In "Discover more marketing tools" card:

How to test the changes in this Pull Request:

Using the WooCommerce Testing Instructions Guide, include your detailed testing instructions:

1. Go to the Marketing page: /wp-admin/admin.php?page=wc-admin&path=%2Fmarketing
2. By referring to the above screenshots, the "Channels" card, "Create a new campaign" modal and "Discover more marketing tools" card should have button text that says "Install extension", instead of "Install plugin".

Changelog entry

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