Don't allow booleans for root proxy config

Setting it to a false or true doesn't affect the config so shouldn't be
allowed. true/false are for role level configurations.

Fixes: https://github.com/basecamp/kamal/issues/1120

test/fixtures/deploy_with_only_workers.yml

@@ -0,0 +1,14 @@
+service: app
+image: dhh/app
+servers:
+  workers:
+    proxy: false
+    hosts:
+      - 1.1.1.1
+      - 1.1.1.2
+primary_role: workers
+registry:
+  username: user
+  password: pw
+builder:
+  arch: amd64