docs: BigQuery package settings added in 0.26.1

[EladBarkay]

Documents three BigQuery changes in dbt package 0.26.1 (CORE-1565) in a new BigQuery settings section of data-tests/dbt/dbt-package.mdx:

• Test table expiration: the new temp_table_expiration_hours var (dbt-data-reliability#1059).
• Nanosecond timestamps: the bigquery_truncate_nanosecond_timestamps var and the one-time fix_nanosecond_timing_values operation, in the order users must run them (dbt-data-reliability#1077, issue [ELE-1434] Make DbtRunner install the dbt packages if needed. #1039).
• Partitioning of test result tables: elementary_test_results and test_result_rows are now partitioned on created_at. Existing tables need a manual rebuild, and a full refresh would delete history (dbt-data-reliability#1057).

Merge only after package 0.26.1 is published, since the page refers to it.

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CORE-1565

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