refactor: Avoid false-positive gcc warning

GCC 14.2.1 will complain about a dangling reference after replacing Span
wiht std::span. This is a false-positive, because std::find does not
return a reference.

Remove the `&` to silence the warning. Also use ranges::find while
touching the line.

src/i2p.cpp:312:21: error: possibly dangling reference to a temporary [-Werror=dangling-reference]
  312 |         const auto& pos = std::find(kv.begin(), kv.end(), '=');
      |                     ^~~
src/i2p.cpp:312:36: note: the temporary was destroyed at the end of the full expression ‘std::find<__gnu_cxx::__normal_iterator<const char*, span<const char> >, char>((& kv)->std::span<const char>::begin(), (& kv)->std::span<const char>::end(), '=')’
  312 |         const auto& pos = std::find(kv.begin(), kv.end(), '=');
      |                           ~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~
cc1plus: all warnings being treated as errors

src/i2p.cpp

@@ -1,4 +1,4 @@
-// Copyright (c) 2020-2022 The Bitcoin Core developers
+// Copyright (c) 2020-present The Bitcoin Core developers
 // Distributed under the MIT software license, see the accompanying
 // file COPYING or http://www.opensource.org/licenses/mit-license.php.
 
@@ -23,6 +23,7 @@
 
 #include <chrono>
 #include <memory>
+#include <ranges>
 #include <stdexcept>
 #include <string>
 
@@ -309,7 +310,7 @@ Session::Reply Session::SendRequestAndGetReply(const Sock& sock,
     reply.full = sock.RecvUntilTerminator('\n', recv_timeout, *m_interrupt, MAX_MSG_SIZE);
 
     for (const auto& kv : Split(reply.full, ' ')) {
-        const auto& pos = std::find(kv.begin(), kv.end(), '=');
+        const auto pos{std::ranges::find(kv, '=')};
         if (pos != kv.end()) {
             reply.keys.emplace(std::string{kv.begin(), pos}, std::string{pos + 1, kv.end()});
         } else {