Fixed defunct hyperlink in PULL_REQUEST_TEMPLATE.md
* Update PULL_REQUEST_TEMPLATE.md
Removed a defunct hyperlink and placed an intuitive directive in its place.
* Updated PULL_REQUEST_TEMPLATE.md to include comment
* Updated PULL_REQUEST_TEMPLATE.md to include lts backporting
+<!-- Comment:
+A great PR typically begins with the line below.
+Replace XXXXX with the numeric part of the issue's id you created on JIRA.
+Please note that if you want your changes backported into LTS, you will need to create a JIRA ticket for it. Read https://jenkins.io/download/lts/#backporting-process for more.
+-->
See [JENKINS-XXXXX](https://issues.jenkins-ci.org/browse/JENKINS-XXXXX).
<!-- Comment:
