fix: resolve Python 3.14 deprecation warnings for asyncio event loop policy

[r266-tech]

Summary

Fixes #3129

Replaces deprecated asyncio.get_event_loop_policy() and asyncio.set_event_loop_policy() calls with sys.version_info checks that use asyncio.set_event_loop_factory() on Python 3.12+ where available.

Changes

sanic/server/loop.py

• try_use_uvloop(): On Python 3.12+ with set_event_loop_factory available, use it instead of the deprecated policy API. Falls back to policy API for older Python versions.
• try_windows_loop(): Same pattern for Windows selector event loop.

sanic/app.py

• get_loop(): Replace asyncio.get_event_loop_policy().get_event_loop() with asyncio.new_event_loop() to avoid deprecated policy API.

Compatibility

• Python 3.10-3.11: Uses existing asyncio.set_event_loop_policy() API (not deprecated in these versions)
• Python 3.12-3.13: Uses policy API (not yet deprecated)
• Python 3.14+: Uses asyncio.set_event_loop_factory() (avoids deprecation warnings)

Testing

The fix uses hasattr(asyncio, 'set_event_loop_factory') as a feature-detection fallback, so it's safe even if the attribute is unexpectedly unavailable.

[r266-tech]

Closing in favor of #3146, which addresses the same issue (#3129) — keeping a single PR open for it. On review, the asyncio.set_event_loop_factory() feature-detection used here never fires on CPython 3.14 (that API does not exist there), so this branch falls back to the policy API and does not actually resolve the warnings. #3146 takes the suppression approach until Python 3.16 forces a real migration. Thanks for the look.