chore(ci): deduplicate CI runs

We don't need to run CI on every push to any branch _and_ PRs targetting
master. This change ensures that the following are targetted:

- New commits to `master`
- PRs targetting `master`

Specifically, commits to new branches are _not_ tested unless they are
part of a PR.

Lastly, removed the `9.x.x` mention as this does not appear to be a
branch that exists.

Signed-off-by: JP-Ellis <[email protected]>

.github/workflows/build-and-test.yml

@@ -2,8 +2,11 @@ name: Build, test, test all examples
 
 on:
   push:
+    branches:
+      - master
   pull_request:
-    branches: [master, 9.x.x]
+    branches:
+      - master
   workflow_dispatch:
 
 env: