Bump nginx versions for CI

Author: davidjb

.github/workflows/build.yml

@@ -14,14 +14,14 @@ jobs:
       matrix:
         include:
           # Mainline
-          - nginx: 1.21.3
+          - nginx: 1.21.4
             dynamic_module: true
-          - nginx: 1.21.3
+          - nginx: 1.21.4
             dynamic_module: false
           # Stable
-          - nginx: 1.20.1
+          - nginx: 1.20.2
             dynamic_module: true
-          - nginx: 1.20.1
+          - nginx: 1.20.2
             dynamic_module: false
           # Past stable versions
           - nginx: 1.18.0