ci: change required labels for issue triage (#38932)

Issue triage _currently_ requires a component to be set and one of the following to be true for an issue to be
considered triaged:
* Marked as a bug _and_ has a severity _and_ has a frequency
* Mark as a feature
* Marked as a refactor
* Marked as a discussion
* Marked as "confusing"
* Marked as "use-case"

This PR changes the rules so that (in addition to the component), triage
requires:
* A priority label (P0 through P5)
* Marked as a feature
* Marked as a discussion

Triage may also apply additional, optional info labels to issues.

[This document outlines the new priority
scheme](https://docs.google.com/document/d/1mN2zWsr1pxChSTHC7UkOgl4PhhuoFONtG_zcMWeqLwA/preview).

While this PR is focused on issue triage and not PR triage, I have
changed the PR section triage to remove reference to the "effort: *" and
"risk: *" labels. Looking through recent PRs, Kapunahele is the only
person applying these, so it's clear that this bit is no longer widely
practiced.

This is just one step in the always-ongoing process of managing GitHub
labels. More adjustments will come over time. In writing this PR, I have
already unearthed a few more areas that can be polished in follow-ups.

PR Close #38932

.github/angular-robot.yml

@@ -137,24 +137,28 @@ triage:
   # arrays of labels that determine if an issue has been fully triaged
   l2TriageLabels:
     -
-      - "type: bug/fix"
-      - "severity*"
-      - "freq*"
+      - "P0"
       - "comp: *"
     -
-      - "type: feature"
+      - "P1"
       - "comp: *"
     -
-      - "type: refactor"
+      - "P2"
       - "comp: *"
     -
-      - "type: RFC / Discussion / question"
+      - "P3"
       - "comp: *"
     -
-      - "type: confusing"
+      - "P4"
       - "comp: *"
     -
-      - "type: use-case"
+      - "P5"
+      - "comp: *"
+    -
+      - "feature"
+      - "comp: *"
+    -
+      - "discussion"
       - "comp: *"
 
 # options for the triage PR plugin