bigquery, bigquery-storage: remove [Beta] from ARROW query results at GA #18803

ARROW query results are marked [Beta] in both libraries. The backend currently marks the feature "This feature is not yet available" (QueryRequest.queryResultsFormat and ArrowSerializationOptions in the REST discovery document, ReadRowsRequest.arrow_serialization_options in storage.proto).

When the feature goes GA, remove [Beta] from:

• google-cloud-bigquery: query_results_format and compression_codec in client.py and _job_helpers.py, and the QueryResultsFormat and QueryResultsCompressionCodec enums in enums.py
• google-cloud-bigquery-storage: read_rows()'s arrow_serialization_options in client.py and the request_template docstring in reader.py (docs(bigquery-storage): mark ReadRows arrow serialization as beta #18802)

timestamp_precision stays [Private Preview] until picosecond timestamps launch.