BigQuery: deprecate client.dataset()

[tswast]

See: #7753

We should recommend the simpler "use a string instead of a reference object" in most cases.

We still call .dataset() 96 times in our samples at docs/snippets.py. I would feel better about adding the deprecation notice after we update the rest of our samples.

[tswast]

As part of these sample updates, I'd also like to move the snippets to their own sample files in the samples/ directory. See: #7683 for examples.

[tswast]

/CC @emar-kar @IlyaFaer

[IlyaFaer]

@tswast, taken to work