Upgrade/Install: Adjust table prefix string check in `setup-config.php` for...

Upgrade/Install: Adjust table prefix string check in `setup-config.php` for the coding standards change in [42343].

Props shashwatmittal, allendav.
Fixes #44318.
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git-svn-id: https://core.svn.wordpress.org/trunk@43479 1a063a9b-81f0-0310-95a4-ce76da25c4cd