Fix DHCP sample gateways and lowercase PXE client-class names

Two sample bugs surfaced while authoring the argocd deployment.

- set subnet gateways to match the authoritative puppet hieradata: 198.18.13-16
  routers are .254 (not .1); 198.18.17 stays .1
- rename KeaClientClass samples Legacy/UEFI-64 to legacy/uefi-64 so they are
  valid RFC1123 object names (the operator renders the kea class name from
  metadata.name, which k8s forces to lowercase); align the README

Claude-Session: https://claude.ai/code/session_01JUoARVdmhxKQHyyyp1pxeT

config/samples/01-subnets.yaml

@@ -10,7 +10,7 @@ spec:
   subnet: 198.18.13.0/24
   pools:
     - 198.18.13.200 - 198.18.13.220
-  routers: [198.18.13.1]
+  routers: [198.18.13.254]
   dnsServers: [198.18.19.15]
   domainName: main.unkin.net
   nextServer: 198.18.19.19
@@ -25,7 +25,7 @@ spec:
   subnet: 198.18.14.0/24
   pools:
     - 198.18.14.200 - 198.18.14.220
-  routers: [198.18.14.1]
+  routers: [198.18.14.254]
   dnsServers: [198.18.19.15]
   domainName: main.unkin.net
   nextServer: 198.18.19.19
@@ -40,7 +40,7 @@ spec:
   subnet: 198.18.15.0/24
   pools:
     - 198.18.15.200 - 198.18.15.220
-  routers: [198.18.15.1]
+  routers: [198.18.15.254]
   dnsServers: [198.18.19.15]
   domainName: main.unkin.net
   nextServer: 198.18.19.19
@@ -55,7 +55,7 @@ spec:
   subnet: 198.18.16.0/24
   pools:
     - 198.18.16.200 - 198.18.16.220
-  routers: [198.18.16.1]
+  routers: [198.18.16.254]
   dnsServers: [198.18.19.15]
   domainName: main.unkin.net
   nextServer: 198.18.19.19