Turn deprecation of applying `@pytest.fixture` to async fixtures in strict mode into an error

[seifertm]

Pytest-asyncio emits a deprecation warning when an async fixture is decorated with @pytest.fixture in strict mode as v0.25 (see #979).

The goal of this issue is to follow through with the deprecation and convert the warning into an error.

This is a breaking change that involves a major version bump to pytest-asyncio.

Please note that the the same behavior has been deprecated in pytest v8.4.0 and will be removed in pytest v9. This means there's a connection to the minimum supported pytest version.