Stop using event_loop fixture (elastic#2969)

It was removed in pytest-asyncio 1.0.

(cherry picked from commit 3c9680a)

# Conflicts:
#	docs/reference/async.md

Author: pquentin

docs/guide/async.asciidoc

@@ -37,8 +37,7 @@ async def main():
     )
     print(resp)
 
-loop = asyncio.get_event_loop()
-loop.run_until_complete(main())
+asyncio.run(main())
 ----
 
 All APIs that are available under the sync client are also available

test_elasticsearch/test_async/test_transport.py

@@ -527,7 +527,8 @@ async def test_sniff_on_node_failure_triggers(self, extra_key, extra_value):
         assert request_failed_in_error
         assert len(client.transport.node_pool) == 3
 
-    async def test_sniff_after_n_seconds(self, event_loop):
+    async def test_sniff_after_n_seconds(self):
+        event_loop = asyncio.get_running_loop()
         client = AsyncElasticsearch(  # noqa: F821
             [NodeConfig("http", "localhost", 9200, _extras={"data": CLUSTER_NODES})],
             node_class=DummyNode,
@@ -581,7 +582,8 @@ async def test_sniffing_disabled_on_elastic_cloud(self, kwargs):
             == "Sniffing should not be enabled when connecting to Elastic Cloud"
         )
 
-    async def test_sniff_on_start_close_unlocks_async_calls(self, event_loop):
+    async def test_sniff_on_start_close_unlocks_async_calls(self):
+        event_loop = asyncio.get_running_loop()
         client = AsyncElasticsearch(  # noqa: F821
             [
                 NodeConfig(