refactor: simplify Obfuscation::HexKey

See: https://github.com/bitcoin/bitcoin/pull/31144#discussion_r2215746554

Co-authored-by: maflcko <6399679+maflcko@users.noreply.github.com>

src/util/obfuscation.h

@@ -78,7 +78,7 @@ public:
 
     std::string HexKey() const
     {
-        return HexStr(std::bit_cast<std::array<uint8_t, KEY_SIZE>>(m_rotations[0]));
+        return HexStr(std::as_bytes(std::span{&m_rotations[0], 1}));
     }
 
 private: