Add delimiters to regular expressions in checkout.php

[becleung]

preg_replace( '[^0-9\.]', '', $_REQUEST['donation'] );

and

preg_replace( '[^0-9\.]', '', $_REQUEST['donation'] );

doesn't do anything anymore. It might have something to do with upgrading to PHP8.

When something like "US$9" is entered into the donation field, it passes right through unchanged.

Adding delimiters gets it working again.

[dparker1005]

Resolves #40