Cleanup dependabot config file + reduced check frequency

This config file is used for feature upgrade checks only, so does not need frequent execution.
Alerts configured in Github directly are intended to trigger checks for security upgrades,
which should happen independent of the schedule defined here.

.github/dependabot.yml

@@ -6,12 +6,8 @@ updates:
   - package-ecosystem: "github-actions"
     directory: "/"
     schedule:
-      interval: "weekly"
+      interval: "monthly"
   - package-ecosystem: "pip"
-    directory: "/pipeline/seq_retrieval/" # Location of package manifests
+    directory: "/pipeline/seq_retrieval/"
     schedule:
-      interval: "weekly"
-  - package-ecosystem: "pip"
-    directory: "/pipeline/seq_retrieval/tests/" # Location of package manifests
-    schedule:
-      interval: "weekly"
+      interval: "monthly"