admin/server: fix rearm_log_level_timer

_log_level_resets is a map<str, level_reset>, we want to extract the
next expiring level_reset to rearm the _log_level_timer.

previously we would extract the min_element based on the key (the logger
name)

so this sequence of actions
set log-level kafka trace 10
set log-level raft trace 10

only kafka would be expired, because once expired the next run would
pick again kafka, see that there is no expiration, and not rearm the
timer.

src/v/redpanda/admin/server.cc

@@ -661,11 +661,17 @@ void admin_server::log_exception(
 void admin_server::rearm_log_level_timer() {
     _log_level_timer.cancel();
 
-    auto next = std::min_element(
-      _log_level_resets.begin(), _log_level_resets.end());
+    if (_log_level_resets.empty()) {
+        return;
+    }
 
-    if (next != _log_level_resets.end() && next->second.expires.has_value()) {
-        _log_level_timer.arm(next->second.expires.value());
+    auto reset_values = _log_level_resets | std::views::values;
+    auto& lvl_rst = *std::ranges::min_element(
+      reset_values, std::less<>{}, [](level_reset const& l) {
+          return l.expires.value_or(ss::timer<>::clock::time_point::max());
+      });
+    if (lvl_rst.expires.has_value()) {
+        _log_level_timer.arm(lvl_rst.expires.value());
     }
 }