Stop using -milestone groups in official projects

$project-milestone groups are a remnant from the past, for
which we are using other names now. Some teams still use them,
though. In an effort to streamline ACLs across official projects,
we'll replace them with the following:

designate-milestone (as used in designate-dashboard stable ACL):
-> use designate-stable-maint instead (already-existing group)

keystone-milestone (as used in keystone pushMerge ACL):
-> use keystone-release instead (already-existing group)

mistral-milestone (as used in mistral stable ACL):
-> rename it to mistral-stable-maint for consistency

This change approval should happen just after mistral-milestone
is manually renamed to mistral-stable-maint, to avoid creating an
extra Gerrit group.

Change-Id: Ib1fb36b4d6e821cd7328efef2ac76e9bcd26dd9a

gerrit/acls/openstack/designate-dashboard.config

@@ -7,9 +7,9 @@ label-Workflow = -1..+1 group designate-core
 label-Workflow = -1..+1 group horizon-core
 
 [access "refs/heads/stable/*"]
-label-Code-Review = -2..+2 group designate-milestone
+label-Code-Review = -2..+2 group designate-stable-maint
 label-Code-Review = -2..+2 group horizon-core
-label-Workflow = -1..+1 group designate-milestone
+label-Workflow = -1..+1 group designate-stable-maint
 label-Workflow = -1..+1 group horizon-core
 
 [receive]

gerrit/acls/openstack/keystone.config

@@ -1,5 +1,5 @@
 [access "refs/for/refs/*"]
-pushMerge = group keystone-milestone
+pushMerge = group keystone-release
 
 [access "refs/heads/*"]
 abandon = group keystone-core

gerrit/acls/openstack/mistral.config

@@ -6,14 +6,14 @@ label-Workflow = -1..+1 group mistral-core
 [access "refs/heads/stable/*"]
 abandon = group Change Owner
 abandon = group Project Bootstrappers
-abandon = group mistral-milestone
+abandon = group mistral-stable-maint
 exclusiveGroupPermissions = abandon label-Code-Review label-Workflow
 label-Code-Review = -2..+2 group Project Bootstrappers
-label-Code-Review = -2..+2 group mistral-milestone
+label-Code-Review = -2..+2 group mistral-stable-maint
 label-Code-Review = -1..+1 group Registered Users
 label-Workflow = -1..+0 group Change Owner
 label-Workflow = -1..+1 group Project Bootstrappers
-label-Workflow = -1..+1 group mistral-milestone
+label-Workflow = -1..+1 group mistral-stable-maint
 
 [receive]
 requireChangeId = true