Bigquery: update bigquery__alter_relation_comment to use alter table ddl instead of API call

[colin-rogers-dbt]

Back in 2018ish when this logic was written BigQuery did not have a DDL syntax for updating the table description. They do now so we should take advantage of it as it will be more performant and more stable.
More performant because our current logic requires two calls: one to get the current table state the other to update it while using the ddl would only require a single call. More stable because it would fix the edge case where an update fails because a concurrent call updated the table in between the get/update calls.

See bigquery__alter_relation_comment code.

[xuliangs]

This is good ticket to tackle. My thought is that we'll be able to deprecate/remove the update_table_description method in the BigQueryAdapter

Then we can in the bigquery__alter_relation_comment to write either ALTER TABLE, ALTER VIEW, or ALTER MATERIALIZED VIEW based on the relation type directly in the Jinja macro

Or

I added this change, if we can assume safely that bigquery__alter_relation_comment will never applies to a non-table-type relation, we can safely use ALTER TABLE