backports: pin using codename

Debian 11 bullseye and Debian 12 bookworm backports release files look
like this:

https://deb.debian.org/debian/dists/bullseye-backports/InRelease:

    Suite: bullseye-backports
    Codename: bullseye-backports

https://deb.debian.org/debian/dists/bookworm-backports/InRelease:

    Suite: stable-backports
    Codename: bookworm-backports

Using "release" (aka Suite in the InRelease file) causes our pin to not
match for bookworm because they changed it to stable-backports instead
of $codename-backports. We can use codename to support both for this
pin.

Fixes #1187.

manifests/backports.pp

@@ -94,11 +94,9 @@
   if $pin =~ Hash {
     $_pin = $pin
   } elsif $pin =~ Numeric or $pin =~ String {
-    # apt::source defaults to pinning to origin, but we should pin to release
-    # for backports
     $_pin = {
       'priority' => $pin,
-      'release'  => $_release,
+      'codename'  => $_release,
     }
   } else {
     fail('pin must be either a string, number or hash')