[ci] update ci esp-dsp test to llvm-21-optimizzation-test

Author: chenqian

.gitlab-ci.yml

@@ -218,7 +218,7 @@ build_and_test:
     expire_in: 1 day
   retry: 2
   variables:
-    TEST_APP_ESP_DSP_CUSTOM_BRANCH: "llvm-20.1.1-optimization-test"
+    TEST_APP_ESP_DSP_CUSTOM_BRANCH: "llvm-21-optimization-test"
   before_script:
     - git config --global --add safe.directory $CI_PROJECT_DIR
   after_script: