[clang-format] Explicitly open DOC_FILE with utf-8 in dump_format_style.py (#79805)

The dump_format_style.py script generates the clang-format style options
documentation.
There was an issue where the script could include spurious characters in
the output when run in windows. It appears that it wasn't defaulting to
the correct encoding when reading the input.
This has been addressed by explicitly setting the encoding when opening
the file.