MNT Update build.yml to push changes from 5.1 branch (#347)

For some reason changing this on the default branch hasn't worked - so lets just change it on all branches that _could_ be relevant, starting with 5.0 and merging up from there.

.github/workflows/build.yml

@@ -5,7 +5,7 @@ on:
     branches:
       - '3'
       - '4.13'
-      - '5.0'
+      - '5.1'
 jobs:
   build:
     name: build-docs