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

@@ -33,8 +33,11 @@ jobs:
           - static
           - dynamic
         nginx-ref:
-          # master
-          - stable-1.26
+          - master
+          - stable-1.28
+        include:
+          - nginx-ref: nginx-1.24
+            module: dynamic
 
     steps:
       - uses: actions/checkout@v4