Remove nested list from GitHub tasklist (#8119)

GitHub task lists do not support indented lists yet

When I submitted https://github.com/jenkinsci/jenkins/pull/8118 , the
task list failed to render until I removed the indented list item

.github/PULL_REQUEST_TEMPLATE.md

@@ -43,8 +43,7 @@ N/A
 ```[tasklist]
 ### Submitter checklist
 - [ ] The Jira issue, if it exists, is well-described.
-- [ ] The changelog entries and upgrade guidelines are appropriate for the audience affected by the change (users or developers, depending on the change) and are in the imperative mood (see [examples](https://github.com/jenkins-infra/jenkins.io/blob/master/content/_data/changelogs/weekly.yml)).
-  - Fill in the **Proposed upgrade guidelines** section only if there are breaking changes or changes that may require extra steps from users during upgrade.
+- [ ] The changelog entries and upgrade guidelines are appropriate for the audience affected by the change (users or developers, depending on the change) and are in the imperative mood (see [examples](https://github.com/jenkins-infra/jenkins.io/blob/master/content/_data/changelogs/weekly.yml)). Fill in the **Proposed upgrade guidelines** section only if there are breaking changes or changes that may require extra steps from users during upgrade.
 - [ ] There is automated testing or an explanation as to why this change has no tests.
 - [ ] New public classes, fields, and methods are annotated with `@Restricted` or have `@since TODO` Javadocs, as appropriate.
 - [ ] New deprecations are annotated with `@Deprecated(since = "TODO")` or `@Deprecated(forRemoval = true, since = "TODO")`, if applicable.