Fix warning about implicit capture of 'this' with '='

The compiler warns about this if you build with
`-Wdeprecated-this-capture`.

The code has been changed in such a way that this issue no longer exists
in 30.0 and newer, so this fix should be merged directly to 29.x instead
of being cherry-picked.

Fixes #22072.

Author: Adam Cozzette

src/google/protobuf/map.cc

@@ -116,7 +116,7 @@ void UntypedMapBase::ClearTable(const ClearInput input) {
   ABSL_DCHECK_NE(num_buckets_, kGlobalEmptyTableSize);
 
   if (alloc_.arena() == nullptr) {
-    const auto loop = [=](auto destroy_node) {
+    const auto loop = [=, this](auto destroy_node) {
       const TableEntryPtr* table = table_;
       for (map_index_t b = index_of_first_non_null_, end = num_buckets_;
            b < end; ++b) {