[style] use constexpr instead of anonymous enum for constants (#11161)

This commit updates a few remaining places where anonymous enums were
used to declare constants, replacing them with `static constexpr`.

tests/unit/test_pool.cpp

@@ -58,10 +58,7 @@ private:
     bool mInitWithInstance;
 };
 
-enum : uint16_t
-{
-    kPoolSize = 11,
-};
+constexpr uint16_t kPoolSize = 11;
 
 typedef Pool<Entry, kPoolSize> EntryPool;