Run Sync e2e tests only on latest version (#3602)

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Task/Issue URL:
https://app.asana.com/0/1204165176092271/1208804974015520/f
Tech Design URL:
CC:

**Description**:
We have experienced reliability issues with e2e tests lately due to CI
instability.
The system seems to be more stable on iOS 17, so this PR temporarily
disable runs on older ones to increase reliability until we figure out a
solution to the CI issues.

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**Steps to test this PR**:
1. Make sure CI is green
- This is the run of sync tests from this branch:
https://github.com/duckduckgo/iOS/actions/runs/11936363646

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.github/workflows/sync-end-to-end.yml

@@ -72,7 +72,7 @@ jobs:
     timeout-minutes: 90
     strategy:
       matrix:
-        os-version: [15, 16, 17]
+        os-version: [17] #[15, 16, 17]
       max-parallel: 1 # Uncomment this line to run tests sequentially. 
       fail-fast: false