Cinc Client: install the current stable release, not a floating pin

The migration script installed Cinc with no version, which floated and
triggered omnitruck's "You are installing a package without a version pin"
warning. Pass -v latest so omnitruck resolves the current stable build for
the node's platform (20.04 -> 19.2.12, 22.04+/24.04/26.04 -> 19.4.46) and
the warning is suppressed. Document the behavior in the README.

Drop the stale chef_version: 18.2.7 pins from the Test Kitchen suites:
dokken defaults chef_version to "latest", so passing -v is what actually
tracks the current stable client.

Add the missing provisioner.product_name: cinc to the deno and strfry
submodule suites so they run Cinc instead of the license-gated Chef Infra
Client.

Submodule pointers:
  site-cookbooks/deno   3795c9e -> c78bc66
  site-cookbooks/strfry 9c04b39 -> 8107b1e

scripts/knife/migrate_node_to_cinc.sh

@@ -8,7 +8,8 @@
 # Idempotent and safe to re-run. Run from the chef repo (uses .cinc/config.rb).
 #
 # What it does on the node (as root, over knife's own SSH connection):
-#   1. installs Cinc Client if the node still runs Chef Infra Client
+#   1. installs the current stable Cinc Client if the node still runs Chef
+#      Infra Client (omnitruck resolves the right build for the platform)
 #   2. makes /etc/cinc resolve to the existing knife-zero config in /etc/chef
 #   3. drops the deprecated attribute whitelist/blacklist lines from client.rb
 #   4. pre-seeds /etc/cinc/client.d/attributes.rb (the allowed_* lists) so the
@@ -41,10 +42,13 @@ remote="$(mktemp)"; trap 'rm -f "$remote"' EXIT
 cat >"$remote" <<EOF
 set -euo pipefail
 
-# 1. Cinc Client (Chef -> Cinc migration, or fresh)
+# 1. Cinc Client (Chef -> Cinc migration, or fresh).
+#    Install the current stable release for this platform. omnitruck resolves the
+#    right build per OS (so 20.04 and 22.04+ each get their latest supported
+#    version), and passing -v also suppresses the "without a version pin" warning.
 if ! command -v cinc-client >/dev/null 2>&1 && [ ! -x /opt/cinc/bin/cinc-client ]; then
-  echo ">> installing Cinc Client"
-  curl -sL https://omnitruck.cinc.sh/install.sh | bash
+  echo ">> installing the current stable Cinc Client"
+  curl -sL https://omnitruck.cinc.sh/install.sh | bash -s -- -v latest
 fi
 
 # 2. Point /etc/cinc at the existing knife-zero config in /etc/chef