WP_Filesystem: FTP Sockets: Avoid using the file_exists() / is_exists() / exists() PemFTP functionality as it's buggy on ncFTPd servers, switching to listing the file instead which is what we use for the FTP Extension transport. Fixes #14049

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Deprecate get_blogaddress_by_domain(), unused since [25261]. props jeremyfelt. fixes #25235.
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