Revert "bump eventlet to latest version that support python 3.12"

This reverts commit 0af727b.

Reason for revert: Seems like this is going to be rolled back. It definitely doesn't pass on downstreams.

Change-Id: Ia2938f6440ea33670a942e0a275c8471326ef800

Author: juliakreger

requirements.txt

@@ -2,7 +2,7 @@
 # of appearance. Changing the order has an impact on the overall integration
 # process, which may cause wedges in the gate later.
 etcd3gw>=2.1.0 # Apache-2.0
-eventlet>=0.34.3 # Apache-2.0
+eventlet>=0.18.2 # Apache-2.0
 stevedore>=1.20.0 # Apache-2.0
 netmiko>=4.1.1 # MIT
 neutron>=13.0.0.0b1 # Apache-2.0