MNT: deprecate round kwarg in Ergast methods (#890)

[divyanshrohatgi]

Follow-up to #920 (the get_event_by_round pilot). Applies the same
round -> round_number deprecation to the Ergast data request methods.
Part of #890.

Changes:

• ergast/interface.py:
  • _build_url (private): renamed round -> round_number. Just a rename,
    it's internal with no external callers.
  • the 12 public get_* methods: added round_number, kept round as a
    deprecated keyword-only alias that emits a FutureWarning, and raise
    ValueError if both are passed. get_lap_times / get_pit_stops also
    raise ValueError if neither is passed (round was required there).
• test_ergast.py: parametrized tests for the deprecated path, the
  both passed case, and the neither passed case. Mocked so they don't
  hit the API.
• test_core.py and the two examples/standings scripts: switched their
  round= calls to round_number= so they don't trip the new warning.
• changelog: Deprecations entry under v3.9.0.
• Also fixed `get_lap_times' docstring summary, which was copy-pasted from
  get_sprint_results.

Left pyproject.toml / the A002 rule out of this on purpose

AI disclosure: used Claude for navigating the codebase and grammar review on this PR description. All code typed and reviewed by me.

[Casper-Guo]

Thanks for your contribution again. This is a bigger diff than last time but the contents are broadly similar. Rest assured that reviewing this is on my todo list.