Add "do not delete this issue template" boilerplate

This was already present on the bug report template. This just adds the
same thing to the enhancement and task templates.

Prompted by #6694

.github/ISSUE_TEMPLATE/enhancement-request.md

@@ -7,6 +7,9 @@ assignees: ''
 
 ---
 
+<!--(Please use this issue template. Do not delete it.)-->
+<!--(Before filing this report, please read: https://www.qubes-os.org/doc/reporting-bugs/)-->
+
 **The problem you're addressing (if any)**
 <!--(A clear and concise description of the problem, if any, that this enhancement is intended to address.)-->
 

.github/ISSUE_TEMPLATE/task.md

@@ -7,6 +7,9 @@ assignees: ''
 
 ---
 
+<!--(Please use this issue template. Do not delete it.)-->
+<!--(Before filing this report, please read: https://www.qubes-os.org/doc/reporting-bugs/)-->
+
 **Qubes OS version (if applicable)**
 <!--(If applicable, the version of Qubes OS that this task concerns (e.g., `R4.0`), available via the command `cat /etc/qubes-release` in a dom0 terminal.)-->