Update merge configurations for release branches (#18945)

Author: T-Gro

.config/service-branch-merge.json

@@ -10,6 +10,10 @@
             "ExtraSwitches": "-QuietComments"
         },
         "release/dev17.14": {
+            "MergeToBranch": "release/dev18.0",
+            "ExtraSwitches": "-QuietComments"
+        },
+        "release/dev18.0": {
             "MergeToBranch": "main",
             "ExtraSwitches": "-QuietComments"
         }