fix: require TLS 1.2 for realtime transport

[subzeroid]

Summary

• require TLS 1.2 or newer for library-created Realtime MQTT SSL contexts
• preserve stricter platform defaults and caller-provided SSL context policies
• mirror the security hardening and regression coverage from instagrapi #2770

Security

Mirrors the fix for instagrapi code scanning alert #18.

Verification

• 596 passed, 3 skipped, 35 subtests passed in the full regression suite
• 24 passed in the focused Realtime regression suite
• Ruff check and format passed
• Bandit reported no issues
• strict MkDocs build passed
• all pre-commit hooks passed
• mypy output is unchanged from the same untouched base environment