🎨 Fix accidental constexpr function

Problem:
- In the trigger scheduler tests, `debug_handler::signal` is accidentally marked
  `constexpr`. Prior to C++23, `constexpr` functions cannot have non-literal
  local variables (like `std::lock_guard`. There is no reason for this function
  to be accidentally C++23 only.

Solution:
- Remove the `constexpr` qualification.

Author: elbeno

test/schedulers/trigger_scheduler.cpp

@@ -170,7 +170,7 @@ struct debug_handler {
     std::mutex m;
 
     template <stdx::ct_string C, stdx::ct_string S, typename Ctx>
-    constexpr auto signal(auto &&...) {
+    auto signal(auto &&...) {
         if constexpr (std::is_same_v<async::debug::tag_of<Ctx>,
                                      async::trigger_scheduler_sender_t>) {
             static_assert(