IO/Stata: clarify unsupported-version error message (text-only change)

[oosei25]

Motivation

Users (esp. students) are often confused when read_stata raises
“Version of given Stata file is {version}” for inputs that aren’t
actually Stata files (e.g., GitHub HTML “blob” pages). The intent of
this PR is to make that message unambiguous without changing any I/O
behavior.

What this changes

• Replace the _version_error message in pandas/io/stata.py with a
  clearer wording that covers both cases:
  • the input is not a valid Stata dataset, or
  • it is a Stata dataset of a version pandas does not support.

No parsing logic changes; only user-facing text.

Tests

Updated/added assertions in pandas/tests/io/test_stata.py to match the
new wording (no behavioral expectations changed):

• test_stata_v117_prefix_with_unsupported_version_raises_version_error

Backward compatibility

• No API/behavior changes.
• Only error text has changed.

Checklist

• Tests updated/passing locally
• Pre-commit checks pass
• User-visible text clarified; no docs/whatsnew needed (happy to add
  a short whatsnew note if maintainers prefer)