UBUNTU: [Debian] Update for leader included in BACKPORT_SUFFIX

UBUNTU: [Debian] Update for leader included in BACKPORT_SUFFIX

BugLink: https://bugs.launchpad.net/bugs/1902957
Currently a ~ is always added to the version string before
BACKPORT_SUFFIX. Now we will also doing forward-ports to
development releases, which works exactly the same as a
backport, but we want to use + as the leader instead.

Our kernel source doesn't contain the information to determine
which leader is appropriate, but that information is available
when generating update.conf. Therefore the leader will be added
as part of BACKPORT_SUFFIX, and our packaging should not insert
any leader.

Signed-off-by: Seth Forshee <seth.forshee@canonical.com>
Acked-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>
Acked-by: Thadeu Lima de Souza Cascardo <cascardo@canonical.com>
Signed-off-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>