feat: remove event type restriction

As far as I can see, there is no reason to limit the source_pr_number
input only to usage with workflow_dispatch events. Other workflows may
also benefit from this input if they want to specify the pull request.

src/backport.ts

@@ -105,15 +105,6 @@ export class Backport {
 
       if (repo === undefined) throw new Error("No repository defined!");
 
-      if (
-        this.config.source_pr_number !== undefined &&
-        this.github.getEventName() !== "workflow_dispatch"
-      ) {
-        throw new Error(
-          "source_pr_number can only be specified for workflow_dispatch events!",
-        );
-      }
-
       const pull_number =
         this.config.source_pr_number === undefined
           ? this.github.getPullNumber()