ui: add the missing label for the backup resource limit

The Resource Limits tab derives each label from the resource type name
returned by listResourceLimits:

    $t('label.max' + item.resourcetypename.replace('_', ''))

Resource.ResourceType declares backup("backup", 12), so the key looked up
for that row is label.maxbackup, which exists in no locale. en.json has
label.maxbackups, a different string used by the backup schedule form for
how many backups to retain, and label.maxbackupstorage, which is resource
type 13. With fallbackLocale 'en' and silentTranslationWarn enabled, the
miss is silent and vue-i18n renders the key itself, so the row reads
"label.maxbackup".

The same key is used for the tagged-limit validation message, which shows
the raw key for the same reason.

Backup was the only one of the 17 ResourceType values without a label; the
other 16 already resolve.

Author: stag7824

ui/public/locales/en.json

@@ -1540,6 +1540,7 @@
 "label.max.primary.storage": "Max. primary (GiB)",
 "label.max.secondary.storage": "Max. secondary (GiB)",
 "label.max.migrations": "Max. migrations",
+"label.maxbackup": "Max. Backups",
 "label.maxbackups": "Max. Backups",
 "label.maxbackupstorage": "Max. Backup Storage (GiB)",
 "label.maxbackups.to.retain": "Max. Backups to retain",