fix: quote "on" key in YAML to prevent boolean parsing

YAML 1.1 reserves "on" as a boolean keyword (meaning True).
Without quotes, Gitea Actions could not parse workflow triggers,
resulting in no jobs being scheduled. This quotes the key as "on":
to ensure it is parsed as a string event trigger key.

.gitea/workflows/ci.yml

@@ -1,6 +1,6 @@
 name: CI/CD Pipeline
 
-on:
+"on":
   push:
     branches: [ master, develop ]
     tags: [ 'v*' ]