cpp-btree: Update to match upstream

Add noexcept to swap-based move constructors and copy/move assignment operator

src/3rdparty/cpp-btree/safe_btree_set.h

@@ -69,13 +69,13 @@ class safe_btree_set : public btree_unique_container<
   }
 
   // Move constructor.
-  safe_btree_set(self_type &&x)
+  safe_btree_set(self_type &&x) noexcept
       : super_type() {
     this->swap(x);
   }
 
   // Copy/move assignment
-  self_type& operator=(self_type x) {
+  self_type& operator=(self_type x) noexcept {
     this->swap(x);
     return *this;
   }