Fix calls to deprecated strings factory API in examples. (#14838)

Follow-up PR to #14771.

I noticed the strings example code still had a deprecated function call:

```
-- Build files have been written to: /opt/conda/conda-bld/work/cpp/examples/strings/build
[1/8] Building CXX object CMakeFiles/libcudf_apis.dir/libcudf_apis.cpp.o
[2/8] Linking CXX executable libcudf_apis
[3/8] Building CUDA object CMakeFiles/custom_prealloc.dir/custom_prealloc.cu.o
[4/8] Building CUDA object CMakeFiles/custom_with_malloc.dir/custom_with_malloc.cu.o
[5/8] Linking CUDA executable custom_prealloc
[6/8] Linking CUDA executable custom_with_malloc
[7/8] Building CUDA object CMakeFiles/custom_optimized.dir/custom_optimized.cu.o
/opt/conda/conda-bld/work/cpp/examples/strings/custom_optimized.cu: In function 'std::unique_ptr<cudf::column> redact_strings(const cudf::column_view&, const cudf::column_view&)':
/opt/conda/conda-bld/work/cpp/examples/strings/custom_optimized.cu:158:40: warning: 'std::unique_ptr<cudf::column> cudf::make_strings_column(cudf::size_type, rmm::device_uvector<int>&&, rmm::device_uvector<char>&&, rmm::device_buffer&&, cudf::size_type)' is deprecated [-Wdeprecated-declarations]
  158 |   auto result =
      |               ~                        ^                                                          
/opt/conda/conda-bld/work/cpp/include/cudf/column/column_factories.hpp:510:42: note: declared here
  510 | [[deprecated]] std::unique_ptr<column> make_strings_column(size_type num_strings,
      |                                          ^~~~~~~~~~~~~~~~~~~
[8/8] Linking CUDA executable custom_optimized
```

Authors:
  - Bradley Dice (https://github.com/bdice)

Approvers:
  - David Wendt (https://github.com/davidwendt)
  - Vyas Ramasubramani (https://github.com/vyasr)
  - Mark Harris (https://github.com/harrism)

URL: #14838

cpp/examples/strings/custom_optimized.cu

@@ -155,8 +155,7 @@ std::unique_ptr<cudf::column> redact_strings(cudf::column_view const& names,
     *d_names, *d_visibilities, offsets.data(), chars.data());
 
   // create column from offsets and chars vectors (no copy is performed)
-  auto result =
-    cudf::make_strings_column(names.size(), std::move(offsets), std::move(chars), {}, 0);
+  auto result = cudf::make_strings_column(names.size(), std::move(offsets), chars.release(), {}, 0);
 
   // wait for all of the above to finish
   stream.synchronize();