Switch system-tests to python 3.12 (#5777)

## Summary of changes

Switch system-tests to python 3.12

## Reason for change

System tests now use python 3.12

## Implementation details

## Test coverage

## Other details
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Author: cbeauchesne

.azure-pipelines/ultimate-pipeline.yml

@@ -4568,8 +4568,8 @@ stages:
 
       - task: UsePythonVersion@0
         inputs:
-            versionSpec: '3.9'
-        displayName: Install python 3.9
+            versionSpec: '3.12'
+        displayName: Install python 3.12
 
       - script: git clone --depth 1 https://github.com/DataDog/system-tests.git
         displayName: Get system tests repo