Add LTS label instructions to PR template (#27924)

* Add LTS explanation and clarify other label explanations

* Link to doc containing LTS calendar

* Change order for simpler cognitive load

* A bit simpler based on feedback

.github/pull_request_template.md

@@ -2,11 +2,9 @@
 What does this PR do?
 
 ### TODO only if you're a HashiCorp employee
-- [ ] **Labels:** If this PR is the CE portion of an ENT change, and that ENT change is
-  getting backported to N-2, use the new style `backport/ent/x.x.x+ent` labels
-  instead of the old style `backport/x.x.x` labels.
-- [ ] **Labels:** If this PR is a CE only change, it can only be backported to N, so use
-  the normal `backport/x.x.x` label (there should be only 1).
+- [ ] **Backport Labels:** If this PR is in the ENT repo and needs to be backported, backport  
+  to N, N-1, and N-2, using the `backport/ent/x.x.x+ent` labels. If this PR is in the CE repo, you should only backport to N, using the `backport/x.x.x` label, not the enterprise labels.
+    - [ ] If this fixes a critical security vulnerability or [severity 1](https://www.hashicorp.com/customer-success/enterprise-support) bug, it will also need to be backported to the current [LTS versions](https://developer.hashicorp.com/vault/docs/enterprise/lts#why-is-there-a-risk-to-updating-to-a-non-lts-vault-enterprise-version) of Vault. To ensure this, use **all** available enterprise labels.
 - [ ] **ENT Breakage:** If this PR either 1) removes a public function OR 2) changes the signature
   of a public function, even if that change is in a CE file, _double check_ that
   applying the patch for this PR to the ENT repo and running tests doesn't