Accept maintainer_note parameter in REST runner registration

What does this MR do and why?

Describe in detail what your merge request does and why.

This MR adds support for taking in a maintainer_note field in the runner registration endpoint. A gitlab/gitlab-runner MR will soon take advantage from this addition so as to allow an admin to specify notes to be associated with the new runner.

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Part of #348299 (closed)

ee/app/assets/javascripts/security_dashboard/components/shared/vulnerability_report/vulnerability_counts.vue

@@ -61,6 +61,11 @@ export default {
       }));
     },
   },
+  watch: {
+    severityCounts() {
+      this.$emit('counts-changed', this.severityCounts);
+    },
+  },
 };
 </script>