Fix C++ 20 compilation warning.

Without this, I see:
implicit capture of 'this' with a capture default of '=' is deprecated [-Wdeprecated-this-capture]

PiperOrigin-RevId: 695445366

implementation/class_loader_ref.h

@@ -72,7 +72,7 @@ class ClassLoaderRef : public ClassLoaderImpl<lifecycleType> {
     if constexpr (ParentLoaderForClass<class_loader_v_, class_v>() !=
                   kDefaultClassLoader) {
       ClassRef_t<JniT<jobject, class_v, class_loader_v_, jvm_v_,
-                      0>>::PrimeJClassFromClassLoader([=]() {
+                      0>>::PrimeJClassFromClassLoader([&]() {
         // Prevent the object (which is a runtime instance of a class) from
         // falling out of scope so it is not released.
         LocalObject loaded_class =