chore: show FutureWarning for LEP endpoints usage with use_regional_endpoints=True

[shobsi]

>>> import bigframes.pandas as bpd
>>> bpd.options.bigquery.use_regional_endpoints = True
<stdin>:1: PreviewWarning: Use of regional endpoints is a feature in preview and available only in selected regions and projects.
>>> bpd.options.bigquery.location = "europe-west1"
>>> s = bpd.get_global_session()
.../bigframes/bigframes/session/clients.py:106: FutureWarning: Support for regional endpoints is not yet available in the location
europe-west1 for BigQuery and BigQuery Storage APIs. For the supported
locations and APIs see https://cloud.google.com/bigquery/docs/regional-endpoints.
For other locations and APIs, currently an older, now deprecated locational
endpoints are being used, which requires your project to be allowlisted. In
future version 2.0 onwards the locational endpoints will no longer be
supported automatically when you enable regional endpoints. However, if you
still need them, you will be able to override the endpoints directly by
doing the following:
    bigframes.pandas.options.bigquery.client_endpoints_override = {
        "bqclient": "https://europe-west1-bigquery.googleapis.com",
        "bqconnectionclient": "europe-west1-bigqueryconnection.googleapis.com",
        "bqstoragereadclient": "europe-west1-bigquerystorage.googleapis.com"
    }
  warnings.warn(
Query job ... is DONE.0 Bytes processed.
https://console.cloud.google.com/bigquery?project=...
>>> 

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Fixes internal issue 399101005 🦕

[gemini-code-assist]

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[tswast]

The built-in FutureWarning from Python is intended to be used for this purpose.

Suggested change

	class DeprecationWarning(Warning):
	"""The functionality or support is being deprecated."""

[shobsi]

Removed, thanks!

[tswast]

Let's use the built-in FutureWarning.

Suggested change

	category=bigframes.exceptions.DeprecationWarning,
	category=FutureWarning,

[shobsi]

Done, thanks!