Add a non-const overload of RepeatedPtrField::GetArena and deprecate the const overload.

Motivation: we are planning to make RepeatedPtrField::GetArena non-const in order to support split RepeatedFields.
PiperOrigin-RevId: 507528755

src/google/protobuf/repeated_ptr_field.h

@@ -1183,6 +1183,9 @@ class RepeatedPtrField final : private internal::RepeatedPtrFieldBase {
   iterator erase(const_iterator first, const_iterator last);
 
   // Gets the arena on which this RepeatedPtrField stores its elements.
+  inline Arena* GetArena();
+
+  ABSL_DEPRECATED("This will be removed in a future release")
   inline Arena* GetArena() const;
 
   // For internal use only.
@@ -1544,6 +1547,11 @@ inline void RepeatedPtrField<Element>::SwapElements(int index1, int index2) {
   RepeatedPtrFieldBase::SwapElements(index1, index2);
 }
 
+template <typename Element>
+inline Arena* RepeatedPtrField<Element>::GetArena() {
+  return RepeatedPtrFieldBase::GetArena();
+}
+
 template <typename Element>
 inline Arena* RepeatedPtrField<Element>::GetArena() const {
   return RepeatedPtrFieldBase::GetArena();