ci: add mainline and switch to stable-1.28.

Add dynamic module tests on stable-1.24 (Ubuntu LTS 24.04).

Author: bavshin-f5

.github/workflows/nginx.yaml

@@ -29,12 +29,15 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
+        nginx-ref:
+          - master
+          - stable-1.28
         module:
           - static
           - dynamic
-        nginx-ref:
-          # master
-          - stable-1.26
+        include:
+          - nginx-ref: stable-1.24
+            module: dynamic
 
     steps:
       - uses: actions/checkout@v4