Update PR template to include add to release notes label (#9691)

**What this PR does / why we need it**:

Includes a checklist option to the PR template about the
`add-to-release-notes` label.

**Special notes for your reviewer**:

**Checklist**
- [ ] Reviewed the
[`CONTRIBUTING.md`](https://github.com/grafana/loki/blob/main/CONTRIBUTING.md)
guide (**required**)
- [ ] Documentation added
- [ ] Tests updated
- [ ] `CHANGELOG.md` updated
- [ ] Changes that require user attention or interaction to upgrade are
documented in `docs/sources/upgrading/_index.md`
- [ ] For Helm chart changes bump the Helm chart version in
`production/helm/loki/Chart.yaml` and update
`production/helm/loki/CHANGELOG.md` and
`production/helm/loki/README.md`. [Example
PR](d10549e3ec)

.github/pull_request_template.md

@@ -10,5 +10,6 @@ Fixes #<issue number>
 - [ ] Documentation added
 - [ ] Tests updated
 - [ ] `CHANGELOG.md` updated
+  - [ ] If the change is worth mentioning in the release notes, add `add-to-release-notes` label
 - [ ] Changes that require user attention or interaction to upgrade are documented in `docs/sources/upgrading/_index.md`
 - [ ] For Helm chart changes bump the Helm chart version in `production/helm/loki/Chart.yaml` and update `production/helm/loki/CHANGELOG.md` and `production/helm/loki/README.md`. [Example PR](https://github.com/grafana/loki/commit/d10549e3ece02120974929894ee333d07755d213)