Rename extra module `apache-airflow[gcp]` to `apache-airflow[google]`

[dinigo]

In the official docs as in the development we use the commandpip install apache-airflow[gcp]to install google hocks and operators.

Now in the master (2.0) this extra installs tools for:

• Google Cloud
• Google Suite
• Google Ads
• Google Marketing cloud

Can we replace gcp extra for google name in the documentation and the packaging setup in airflow v2.0?

[potiuk]

Yep. I think it's the right time to do it :)

[ashwinshankar77]

@turbaszek I will take a stab at this one