Enable Dependabot security updates and name a maintainer

.github/CODEOWNERS

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+# Maintainer for this example. Dependabot security PRs request this person's review.
+* @stephen-derosa

.github/dependabot.yml

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+# Security updates only: version-update PRs are off (open-pull-requests-limit: 0)
+# and security fixes are grouped instead of one PR per package. Security PRs open
+# when an advisory lands; the schedule only applies if version updates are turned on.
+version: 2
+updates:
+  - package-ecosystem: github-actions
+    directory: /
+    schedule:
+      interval: quarterly
+    open-pull-requests-limit: 0
+    groups:
+      security:
+        applies-to: security-updates
+        patterns:
+          - "*"
+  - package-ecosystem: pip
+    directory: /src/waver_driver
+    schedule:
+      interval: quarterly
+    open-pull-requests-limit: 0
+    groups:
+      security:
+        applies-to: security-updates
+        patterns:
+          - "*"