Replace "extra" in "the following extra packages [...]" by "additional"

This breaks the translation for no big gain, but we broke enough strings
already for that to not really matter anymore.

Closes: #82430

apt-private/private-install.cc

@@ -682,7 +682,7 @@ bool DoInstall(CommandLine &CmdL)
       to what the user asked */
    SortedPackageUniverse Universe(Cache);
    if (Cache->InstCount() != verset[MOD_INSTALL].size())
-      ShowList(c1out, _("The following extra packages will be installed:"), Universe,
+      ShowList(c1out, _("The following additional packages will be installed:"), Universe,
 	    PkgIsExtraInstalled(&Cache, &verset[MOD_INSTALL]),
 	    &PrettyFullName, CandidateVersion(&Cache));