Remove some references to release/6.0 (#58929)

Author: wtgodbe

.azure/pipelines/helix-matrix.yml

@@ -11,7 +11,6 @@ schedules:
 - cron: "0 9 * * *"
   branches:
     include:
-    - release/6.0
     - release/8.0
     - release/9.0
   always: false

.azure/pipelines/quarantined-pr.yml

@@ -3,7 +3,7 @@
 #
 
 # Configure which branches trigger builds
-# We want to run quarantined tests on release/6.0 and later as well as on PRs
+# We want to run quarantined tests on release/8.0 and later as well as on PRs
 trigger:
   batch: true
   branches:

.github/dependabot.yml

@@ -60,18 +60,6 @@ updates:
     labels:
       - area-infrastructure
     target-branch: "release/2.1"
-  - package-ecosystem: "gitsubmodule"
-    directory: "/"
-    schedule:
-      interval: "monthly"
-    allow:
-      - dependency-type: "all"
-    commit-message:
-      prefix: "[release/6.0] "
-      include: scope
-    labels:
-      - area-infrastructure
-    target-branch: "release/6.0"
   - package-ecosystem: "gitsubmodule"
     directory: "/"
     schedule: