Chore: [AEA-6055] - use latest regression tests (#2971)

## Summary

- Routine Change

### Details

Pick up the latest regression tests, specifically post-dated
notifications added in 3.11.0.

Author: tstephen-nhs

.github/workflows/run_regression_tests.yml

@@ -60,11 +60,11 @@ jobs:
         run: |
           if [[ "$TARGET_ENVIRONMENT" != "prod" && "$TARGET_ENVIRONMENT" != "ref" ]]; then
             # this should be the tag of the tests you want to run
-            REGRESSION_TEST_REPO_TAG=v3.10.9
+            REGRESSION_TEST_REPO_TAG=v3.12.20
 
             # this should be the tag of the regression test workflow you want to run
             # This will normally be the same as REGRESSION_TEST_REPO_TAG
-            REGRESSION_TEST_WORKFLOW_TAG=v3.10.9
+            REGRESSION_TEST_WORKFLOW_TAG=v3.12.20
 
             curl https://raw.githubusercontent.com/NHSDigital/electronic-prescription-service-api-regression-tests/refs/tags/${REGRESSION_TEST_WORKFLOW_TAG}/scripts/run_regression_tests.py -o run_regression_tests.py
             poetry install