fix: Replace asyncio.iscoroutinefunction with compatibility shim for Python 3.16

[Dibbu-cell]

Description

This PR fixes the deprecation warning for asyncio.iscoroutinefunction that will be removed in Python 3.16. The function was deprecated in Python 3.12 in favor of inspect.iscoroutinefunction.

Issues

• resolves: Replace asyncio.iscoroutinefunction #4900

Reminders

• Please add tests to validate your changes, and lint your code using tox -e linters.
• Add GH Issue ID & Linear ID (if applicable)
• PR title should use conventional commit style (feat:, fix:, ref:, meta:)
• For external contributors: CONTRIBUTING.md, Sentry SDK development docs, Discord community

[seer-by-sentry]

Potential bug: The fallback for markcoroutinefunction uses asyncio.coroutines._is_coroutine, which was removed in Python 3.11, causing an AttributeError.

• Description: The compatibility shim for markcoroutinefunction provides a fallback for Python versions older than 3.12. This fallback incorrectly assumes asyncio.coroutines._is_coroutine exists. However, this attribute was removed in Python 3.11. When the SDK is used with a Django ASGI application on Python 3.11, the fallback logic is triggered, leading to an AttributeError when markcoroutinefunction is called in sentry_sdk/integrations/django/asgi.py.

• Suggested fix: The fallback implementation for markcoroutinefunction needs to be updated to not use asyncio.coroutines._is_coroutine. A different implementation that is compatible with Python versions prior to 3.12, including 3.11, should be used.
  _{severity: 0.85, confidence: 0.95}

_{Did we get this right? 👍 / 👎 to inform future reviews.}