pw_thread_freertos: adjust detach to CHECK joinable

Adjusts the FreeRTOS backend to use PW_CHECK(joinable()) in
detach just like ThreadX's backend.

No-Docs-Update-Reason: No API change.
Change-Id: Idd341288d8616819b2d1fa6da203b91b5c54d68d
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/40666
Commit-Queue: Auto-Submit <auto-submit@pigweed.google.com.iam.gserviceaccount.com>
Reviewed-by: Wyatt Hepler <hepler@google.com>
Pigweed-Auto-Submit: Ewout van Bekkum <ewout@google.com>
diff --git a/pw_thread_freertos/thread.cc b/pw_thread_freertos/thread.cc
index 665415b..b77df78 100644
--- a/pw_thread_freertos/thread.cc
+++ b/pw_thread_freertos/thread.cc
@@ -179,7 +179,7 @@
 }
 
 void Thread::detach() {
-  PW_DCHECK(joinable());
+  PW_CHECK(joinable());
 
 #if INCLUDE_vTaskSuspend == 1
   // No need to suspend extra tasks.