Back-patch pg_encoding_verifymbstr()/pg_encoding_verifymbchar() to v13.

Back-patch pg_encoding_verifymbstr()/pg_encoding_verifymbchar() to v13.

A security fix will need those functions, so back-patch the v14+ functions to
v13.

When commit b80e10638e36b9d2f0b39170c613837af2ca2aac introduced the v14+
implementation of pg_encoding_verifymbstr(), it added a callback to each
pg_wchar_table entry.  For simplicity and ABI stability, this instead
implements the function in terms of the existing per-character callback.

Author: Noah Misch <noah@leadboat.com>
Author: Andres Freund <andres@anarazel.de>
Security: CVE-2025-1094