update google-cloud-bigquery to use `bigquery_magics` package

[tswast]

The code in https://github.com/googleapis/python-bigquery/tree/main/google/cloud/bigquery/magics is redundant with this module. After our first release, delete that code and update google-cloud-bigquery to be backwards-compatible.

Sub-tasks:

• [x] Update google.cloud.bigquery.load_ipython_extension to load the bigquery_magics extension instead. Raise an ~exception~ warning prompting the user to install bigquery_magics if not installed.
• [N/A] ~Remove https://github.com/googleapis/python-bigquery/tree/main/google/cloud/bigquery/magics except for google.cloud.bigquery.magics.context and google.cloud.bigquery.magics.magics.context, which will be aliases to the bigquery_magics.context object.~ Edit: keeping this code intact for now, but deprecating it. To be removed in a future version of google-cloud-bigquery.
• [N/A] ~Update unit/system tests to be minimal to verify that we're calling out to bigquery_magics now.~ keeping this code intact for now, but deprecating it. To be removed in a future version of google-cloud-bigquery.
• [x] Update the magics-related "extras" in google-cloud-bigquery's setup.py to just depend on bigquery_magics.

[tswast]

https://github.com/googleapis/python-bigquery/pull/1965 covers this issue. Note: it doesn't fully remove the google.cloud.bigquery.magics module, just deprecates it and updates to use this package when available.