fuzz: replace hardcoded numbers for bech32 limits

Use bech32::CharLimit::BECH32 and bech32::CHECKSUM_SIZE instead of
hardcoded values. This is a follow-up fix for #34007
(where this file was missed)

src/bech32.cpp

@@ -19,9 +19,6 @@ namespace
 
 typedef std::vector<uint8_t> data;
 
-/** The Bech32 and Bech32m checksum size */
-constexpr size_t CHECKSUM_SIZE = 6;
-
 /** The Bech32 and Bech32m character set for encoding. */
 const char* CHARSET = "qpzry9x8gf2tvdw0s3jn54khce6mua7l";