[Fleet] Add upgrade_attempts to .fleet-agents index (#123256)

* [Fleet] Add upgrade_attemps to .fleet-agents index

* Change type to date

---------

Co-authored-by: Elastic Machine <[email protected]>

Author: jillguyonnet

x-pack/plugin/core/template-resources/src/main/resources/fleet-agents.json

@@ -310,6 +310,9 @@
         "upgrade_status": {
           "type": "keyword"
         },
+        "upgrade_attempts": {
+          "type": "date"
+        },
         "user_provided_metadata": {
           "type": "object",
           "enabled": false