Deprecated func fix rename fix

[elvismdev]

Description of the Change

• Closes https://github.com/10up/MU-Migration/pull/95

How to test the Change

Run a single site to multisite import. No PHP deprecated warnings should output to the console. No PHP warnings when moving plugins and themes file assets.

Changelog Entry

Fixed - Deprecated WordPress function warning, insert_blog() with wp_insert_site(). Fixed - PHP warning when calling replace() - caused by PHP core bug https://bugs.php.net/bug.php?id=54097

Credits

Props @elvismdev

Checklist:

• [x] I agree to follow this project's Code of Conduct.
• [ ] I have updated the documentation accordingly.
• [ ] I have added tests to cover my change.
• [ ] All new and existing tests pass.