[Renovate/Core] Use prev-minor backport strategy (elastic#204709)

## Summary

Speaking to @azasypkin, we noticed that we are not backporting many of
our dependency updates. These were mostly set to `backport:skip` before
the 9 vs. 8.x branch split. But we need it at the moment.

We'll need to revisit our backport strategy once we release 9.0.

### Identify risks

- [ ] Some deps like `@elastic/elasticsearch` might be bumped to 9.0 and
shouldn't be backported. We'll update the renovate file when that
occurs.

renovate.json

@@ -519,7 +519,7 @@
       ],
       "labels": [
         "release_note:skip",
-        "backport:skip",
+        "backport:prev-minor",
         "Team:Operations",
         "Team:Core"
       ],
@@ -610,7 +610,7 @@
       "labels": [
         "release_note:skip",
         "Team:Core",
-        "backport:skip"
+        "backport:prev-minor"
       ],
       "enabled": true
     },
@@ -628,7 +628,7 @@
       "labels": [
         "release_note:skip",
         "Team:Core",
-        "backport:skip"
+        "backport:prev-minor"
       ],
       "enabled": true
     },
@@ -665,7 +665,7 @@
       "labels": [
         "release_note:skip",
         "Team:Core",
-        "backport:all-open"
+        "backport:prev-minor"
       ],
       "enabled": true
     },
@@ -684,7 +684,7 @@
       "labels": [
         "release_note:skip",
         "Team:Core",
-        "backport:skip"
+        "backport:prev-minor"
       ],
       "enabled": true
     },
@@ -704,7 +704,7 @@
       "labels": [
         "release_note:skip",
         "Team:Core",
-        "backport:all-open"
+        "backport:prev-minor"
       ],
       "minimumReleaseAge": "7 days",
       "enabled": true
@@ -2965,4 +2965,4 @@
       "datasourceTemplate": "docker"
     }
   ]
-}
+}