chore(deps): update dependency google-cloud-bigquery to v2

[renovate-bot]

This PR contains the following updates:

Package	Update	Change
google-cloud-bigquery	major	==1.26.1 -> ==2.0.0

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Release Notes

googleapis/python-bigquery

v2.0.0

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09-30-2020 14:51 PDT

Implementation Changes

• Transition the library to microgenerator. (#​278)
  This is a breaking change that drops support for Python 2.7 and 3.5 and brings a few other changes.
  See migration guide for more info.

Internal / Testing Changes

• Update protoc-generated comments (via synth). (#​270)
• Add CI secrets manager (via synth). (#​271)

v1.28.0

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Features

• add custom cell magic parser to handle complex --params values (#​213) (dcfbac2)
• add instrumentation to list methods (#​239) (fa9f9ca)
• add opentelemetry tracing (#​215) (a04996c)
• expose require_partition_filter for hive_partition (#​257) (aa1613c)

Bug Fixes

• fix dependency issue in fastavro (#​241) (2874abf)
• update minimum dependency versions (#​263) (1be66ce)
• validate job_config.source_format in load_table_from_dataframe (#​262) (6160fee)

Documentation

• recommend insert_rows_json to avoid call to tables.get (#​258) (ae647eb)

1.27.2 (2020-08-18)

Bug Fixes

• rationalize platform constraints for 'pyarrow' extra (#​235) (c9a0567)

1.27.1 (2020-08-18)

Bug Fixes

• tweak pyarrow extra to soothe PyPI (#​230) (c15efbd)

v1.27.2

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Closes#288 288

[tswast]

@crwilcox Apparently I'm not a codeowner for files in the samples directory. Could you merge?