cache: deprecated API first_order_merge (#84568)

* deprecated legacy method first_order_merge

Signed-off-by: Abhijeet Kasurde <akasurde@redhat.com>

changelogs/fragments/deprecate_api.yml

@@ -0,0 +1,3 @@
+---
+deprecated_features:
+  - fact_cache - deprecate first_order_merge API (https://github.com/ansible/ansible/pull/84568).

lib/ansible/vars/fact_cache.py

@@ -58,6 +58,10 @@ class FactCache(MutableMapping):
         self._plugin.flush()
 
     def first_order_merge(self, key, value):
+        display.deprecated(
+            "API 'first_order_merge' is deprecated, please update the usage",
+            version="2.22"
+        )
         host_facts = {key: value}
 
         try: