Drop lower-constraints.txt and its testing

As discussed in TC PTG[1] and TC resolution[2], we are
dropping the lower-constraints.txt file and its testing.
We will keep lower bounds in the requirements.txt file but
with a note that these are not tested lower bounds and we
try our best to keep them updated.

NOTE(elod.illes): setting neutron-tempest-plugin-dynamic-routing as
non-votin for now to unblock the gate, as the job is failing. This can
be set back as voting if a fix is found to this issue.

[1] https://etherpad.opendev.org/p/tc-zed-ptg#L326
[2] https://governance.openstack.org/tc/resolutions/20220414-drop-lower-constraints.html#proposal

Change-Id: I2a42a907170a6fcd8cae245f864121fd12422ef0
(cherry picked from commit 712535f6be)

.zuul.yaml

@@ -7,7 +7,8 @@
     check:
       jobs:
         - tempest-integrated-networking
-        - neutron-tempest-plugin-dynamic-routing
+        - neutron-tempest-plugin-dynamic-routing:
+            voting: false
     gate:
       jobs:
         - tempest-integrated-networking

requirements.txt

@@ -1,3 +1,7 @@
+# Requirements lower bounds listed here are our best effort to keep them up to
+# date but we do not test them so no guarantee of having them all correct. If
+# you find any incorrect lower bounds, let us know or propose a fix.
+
 # The order of packages is significant, because pip processes them in the order
 # of appearance. Changing the order has an impact on the overall integration
 # process, which may cause wedges in the gate later.