Parse standard cookie Expires dates from Morsel

[lin-hongkuan]

Summary

• parse Morsel expires values with http.cookiejar.http2time instead of a single fixed strptime template
• keep invalid expires values rejected when the stdlib parser cannot parse them
• add regression coverage for RequestsCookieJar.update(SimpleCookie(...)) with a standard HTTP-date Expires value

Fixes #6004

Testing

• python -c "import sys, pytest; sys.path.insert(0, 'src'); raise SystemExit(pytest.main(['tests/test_requests.py::TestMorselToCookieExpires', '-q']))"
• python -m ruff check src/requests/cookies.py tests/test_requests.py
• git diff --check