chore: label incoming issues with a triage label (#8962)

.github/ISSUE_TEMPLATE/bug_report.md

 ---
 name: Bug report
 about: You've found a bug with Renovate
-labels: 'type:bug, status:requirements'
+labels: 'type:bug, status:requirements, priority-5-triage'
 ---
 
 <!--

.github/ISSUE_TEMPLATE/feature_request.md

 ---
 name: Feature request
 about: Suggest an idea for this project
-labels: 'type:feature, status:requirements'
+labels: 'type:feature, status:requirements, priority-5-triage'
 ---
 
 **What would you like Renovate to be able to do?**

docs/development/issue-labeling.md

@@ -63,14 +63,18 @@ Add the `breaking` label for Issues or PRs which contain changes that are not ba
     priority-2-important
     priority-3-normal
     priority-4-low
+    priority-5-triage
 
 </details>
 
 Use these to assign a priority level to an issue.
+Incoming issues are labeled `priority-5-triage` by default, this label should be replaced with a proper priority (low/normal/important/critical).
 Make a best-effort attempt to select a proper priority.
 Nothing bad will happen if you select a "wrong" priority.
 At a high level: critical = needs immediate fix, important = to be prioritized ahead of others, normal = default priority, low = trivial issue, or impacts a very small % of the user base.
 
+Use [this search](https://github.com/renovatebot/renovate/issues?q=is%3Aissue+is%3Aopen+sort%3Aupdated-desc+-label%3Apriority-1-critical+-label%3Apriority-2-important+-label%3Apriority-3-normal+-label%3Apriority-4-low++-label%3Apriority-5-triage) to find any issues which are missing a priority label.
+
 ### Platform
 
 <details>