CI: miscellaneous cleanups (#8073)

## Problem
There are a couple of small CI cleanups that seem too small for dedicated PRs

## Summary of changes
- Create release PR with the title that matches the title in the description
- Tune error message for disallowing `ubuntu-latest` to explicitly
mention what to do
- Remove junit output from pytest, we use allure instead

.github/actions/run-python-test-set/action.yml

@@ -183,8 +183,7 @@ runs:
 
         # Run the tests.
         #
-        # The junit.xml file allows CI tools to display more fine-grained test information
-        # in its "Tests" tab in the results page.
+        # --alluredir saves test results in Allure format (in a specified directory)
         # --verbose prints name of each test (helpful when there are
         # multiple tests in one file)
         # -rA prints summary in the end
@@ -193,7 +192,6 @@ runs:
         #
         mkdir -p $TEST_OUTPUT/allure/results
         "${cov_prefix[@]}" ./scripts/pytest \
-          --junitxml=$TEST_OUTPUT/junit.xml \
           --alluredir=$TEST_OUTPUT/allure/results \
           --tb=short \
           --verbose \