fleetctl gitops --dry-run now errors on duplicate (or conflicting) global/team enroll secrets. (#19344)

#19152
`fleetctl gitops --dry-run` now errors on duplicate (or conflicting)
global/team enroll secrets.

- One check is done on the backend to check against existing secrets
- Another check is done in fleetctl to detect duplicate secrets coming
in

# Checklist for submitter

- [x] Changes file added for user-visible changes in `changes/`,
`orbit/changes/` or `ee/fleetd-chrome/changes`.
See [Changes
files](https://fleetdm.com/docs/contributing/committing-changes#changes-files)
for more information.
- [x] Added/updated tests
- [x] Manual QA for all new/changed functionality

cmd/fleetctl/testdata/gitops/team_config_no_paths.yml

@@ -1,8 +1,8 @@
 name: "${TEST_TEAM_NAME}"
 team_settings:
   secrets:
-    - secret: "SampleSecret123"
-    - secret: "ABC"
+    - secret: "SampleSecret123-team"
+    - secret: "ABC-team"
   webhook_settings:
     host_status_webhook:
       days_count: 14