Advise users to retain issue/PR templates

Advise users to retain issue/PR templates

Occasionally we get issues and PRs from users who delete the
templates.  Advise users that their issues and PRs may be closed if
they do not fill out the templates as we really need this information.

Also updating PR template to drop unneeded approval toggle as we are
now using issue labels for status tracking.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed-by: Matthew Ahrens <mahrens@delphix.com>
Signed-off-by: bunder2015 <omfgbunder@gmail.com>
Closes #8029