[air] Upgrade release tests that depend on upgraded modin to py38 (ra…

…y-project#36402)

ray-project#30895 upgraded the pinned version of modin, removing support for python <= 3.7. All the release tests run with py37 by default, so the cluster env was failing to build due to no compatible modin version. This PR runs these release tests with py38 instead, which aligns with the us moving to testing with py38 for all of CI.

Signed-off-by: Justin Yu <justinvyu@anyscale.com>
Signed-off-by: e428265 <arvind.chandramouli@lmco.com>

release/release_tests.yaml

@@ -213,7 +213,7 @@
 - name: air_benchmark_xgboost_cpu_10
   group: AIR tests
   working_dir: air_tests/air_benchmarks
-
+  python: "3.8"
   frequency: nightly
   team: ml
 
@@ -994,7 +994,7 @@
 - name: xgboost_train_gpu
   group: XGBoost
   working_dir: xgboost_tests
-
+  python: "3.8"
   frequency: nightly
   team: ml