Update dependabot.yaml for security updates

Testing if dependabot can create security pull request for release-2.6, 2.7 and 2.8 branches.

I am trying to cheat with a separate `updates` entry per branch. 

Most likely dependabot cannot bump only security-relevan dependencies in older branches, see  dependabot/dependabot-core#2767 (comment)

.github/dependabot.yaml

@@ -18,3 +18,63 @@ updates:
     - "release-note-none"
     - "ok-to-test"
   open-pull-requests-limit: 10
+
+# Experimenting with security fixes to 2.6, 2.7 and 2.8 branches
+- package-ecosystem: gomod
+  directory: "/"
+  schedule:
+    interval: daily
+  labels:
+    - "area/dependency"
+    - "release-note-none"
+    - "ok-to-test"
+  open-pull-requests-limit: 10
+- package-ecosystem: "github-actions"
+  directory: "/"
+  schedule:
+      interval: "daily"
+  labels:
+    - "area/dependency"
+    - "release-note-none"
+    - "ok-to-test"
+  open-pull-requests-limit: 0
+  target-branch: "release-2.6"
+
+- package-ecosystem: gomod
+  directory: "/"
+  schedule:
+    interval: daily
+  labels:
+    - "area/dependency"
+    - "release-note-none"
+    - "ok-to-test"
+  open-pull-requests-limit: 10
+- package-ecosystem: "github-actions"
+  directory: "/"
+  schedule:
+      interval: "daily"
+  labels:
+    - "area/dependency"
+    - "release-note-none"
+    - "ok-to-test"
+  open-pull-requests-limit: 0
+  target-branch: "release-2.7"
+- package-ecosystem: gomod
+  directory: "/"
+  schedule:
+    interval: daily
+  labels:
+    - "area/dependency"
+    - "release-note-none"
+    - "ok-to-test"
+  open-pull-requests-limit: 10
+- package-ecosystem: "github-actions"
+  directory: "/"
+  schedule:
+      interval: "daily"
+  labels:
+    - "area/dependency"
+    - "release-note-none"
+    - "ok-to-test"
+  open-pull-requests-limit: 0
+  target-branch: "release-2.8"