fix: translation string

[Soare-Robert-Daniel]

Summary

Changes:

• add missing translator comments
• add localization for upgrade URLs

Generated .pot file: feedzy-free.pot.zip (with command wp i18n make-pot . languages/feedzy-free.pot)

Will affect visual aspect of the product

NO

Test instructions

• Check if the strings and upgrade URL are in the correct format.

Check before Pull Request is ready:

• [ ] I have written a test and included it in this PR
• [x] I have run all tests and they pass
• [x] The code passes when running the PHP CodeSniffer
• [x] Code meets WordPress Coding Standards for PHP, HTML, CSS and JS
• [x] Security and Sanitization requirements have been followed
• [ ] I have assigned a reviewer or two to review this PR (if you're not sure who to assign, we can do this step for you)

Closes https://github.com/Codeinwp/feedzy-rss-feeds/issues/978

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:tada: This PR is included in version 4.4.14 :tada:

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