group filter: rename CLI/API/Config "groups" option to "group-filter"

we even use that for basically all the related schema names, "groups"
allone is just rather not so telling, i.e., "groups" what?

While due to the additive nature of `group-filter` is not the best
possible name for passing multiple arguments on the CLI (the web-ui
can present this more UX-friendly anyway) due to possible confusion
about if the filter act like AND vs OR it can be documented and even
if a user is confused they still are safe on more being synced than
less. Also, the original param name wasn't really _that_ better in
that regards

Dietmar also suggested to use singular for the CLI option, while
there can be more they're passed over repeating the option, each with
a single filter.

Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>

src/api2/tape/backup.rs

@@ -437,7 +437,7 @@ fn backup_worker(
 
     group_list.sort_unstable();
 
-    let (group_list, group_count) = if let Some(group_filters) = &setup.groups {
+    let (group_list, group_count) = if let Some(group_filters) = &setup.group_filter {
         let filter_fn = |group: &BackupGroup, group_filters: &[GroupFilter]| {
             group_filters.iter().any(|filter| group.matches(filter))
         };