chore: make dependabot update nested lock files as well (#275)

Found that dependabot wasn't bumping Ruby SDK lockfile. Adding
ruby-sdk directory explicitly should fix that.

Note that this does remove root directory, so the root lockfile will
not get updated. That should be fine as it is unused for releases (and
we should probably remove it altogether libraries shouldn't normally
have lockfiles committed).

Author: rasendubi

.github/dependabot.yml

@@ -11,11 +11,15 @@ updates:
       interval: "daily"
 
   - package-ecosystem: cargo
-    directory: "/"
+    directories:
+      - "/eppo_core"
+      - "/rust-sdk"
+      - "/python-sdk"
+      - "/ruby-sdk"
+      - "/elixir-sdk"
     schedule:
       interval: "weekly"
 
-
   - package-ecosystem: pip
     directory: "/python-sdk"
     schedule: