[3008.x] Replace deprecated asyncio.iscoroutinefunction with inspect.iscoroutinefunction (#69580)

[dwoz]

What does this PR do?

Backport of #69581 to 3008.x.

asyncio.iscoroutinefunction() was deprecated in Python 3.12 and is slated for removal in Python 3.16. This PR replaces it with inspect.iscoroutinefunction() in:

• salt/utils/event.py
• salt/cluster/consensus/raft/scheduler.py

Tests are added that verify no DeprecationWarning is emitted when scheduling coroutine callbacks through SaltEvent._schedule and AsyncTimeoutScheduler.schedule.

Why a sibling PR?

The original PR #69581 was merged to master (Potassium), but issue #69580 explicitly states:

We need to ensure we stop using deprecated functions on 3008.x.

So the master merge alone does not address the issue. This PR is the sibling backport that lands the same fix on 3008.x (Argon).

Related

• Fixes [bug] fix deprecation warning #69580
• Original master PR: Replace deprecated asyncio.iscoroutinefunction with inspect.iscoroutinefunction #69581 (merge commit `8b6c861d4df1a44a66be3cf838788d8b52bea393`)

Commits

• Cherry-pick of `e70647eca19b2150098ae29c49ea29b127fc60c1` applies cleanly on `3008.x` with no conflicts. Both `salt/cluster/consensus/raft/scheduler.py` and its test exist on 3008.x, so the full set of files from the master PR was applied (nothing skipped).

Test plan

• `venv310/bin/pytest tests/pytests/unit/utils/event/test_salt_event_schedule.py -v` -> 2 passed
• `venv310/bin/pytest tests/pytests/unit/cluster/consensus/test_raft_scheduler.py -v` -> 16 passed
• `pre-commit run --files ` clean