Deprecate and remove `AsyncRowReader` from the public API

[dbolduc]

The public part of this class is just one static variable.

google-cloud-cpp/google/cloud/bigtable/async_row_reader.h

Lines 47 to 57 in a24af94

	template <typename RowFunctor, typename FinishFunctor>
	class AsyncRowReader : public std::enable_shared_from_this<
	AsyncRowReader<RowFunctor, FinishFunctor>> {
	public:
	/// Special value to be used as rows_limit indicating no limit.
	// NOLINTNEXTLINE(readability-identifier-naming)
	static std::int64_t constexpr NO_ROWS_LIMIT = 0;
	// Callbacks keep pointers to these objects.
	AsyncRowReader(AsyncRowReader&&) = delete;
	AsyncRowReader(AsyncRowReader const&) = delete;

I think we should just deprecate it and point customers to RowReader::NO_ROWS_LIMIT.

google-cloud-cpp/google/cloud/bigtable/row_reader.h

Line 58 in a24af94

static std::int64_t constexpr NO_ROWS_LIMIT = 0;

(The AsyncRowReader implementation gets moved into bigtable/internal/async_row_reader.h)

[coryan]

These are deprecated, scheduled for removal around 2023-07.

[mpeddada1]

Fixed in #15842