Chore: deprecate the current default user-agent string.

[chalmerlowe]

The default user-agent issued by this module is not in accordance with RFC 9110 which recommends: product/major.minor.micro (ie. pandas/2.0.1).

Current style: product-x.y.z (pandas-2.0.1)

• [ ] Deprecate the existing user agent string and replace it with a correctly formatted user agent.
• [ ] Change the default setting for rfc9110_delimiter in the create_user_agent() method to True.

Note: in the docstring for user_agent in the Google api-core ClientInfo class, the api-core folks recommend but do not require the use of the format the rfc suggests (with the forward slash):

user_agent (Optional[str]): Prefix to the user agent header. This is
            used to supply information such as application name or partner tool.
            Recommended format: ``application-or-tool-ID/major.minor.version``.