chore: setup release CI and mergify for version-16 (#35859)

Signed-off-by: Akhil Narang <me@akhilnarang.dev>

.github/workflows/initiate_release.yml

@@ -15,7 +15,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        version: ["14", "15"]
+        version: ["14", "15", "16"]
 
     steps:
       - uses: octokit/request-action@v2.x

.mergify.yml

@@ -31,16 +31,6 @@ pull_request_rules:
         assignees:
           - "{{ author }}"
 
-  - name: backport to version-13-hotfix
-    conditions:
-      - label="backport version-13-hotfix"
-    actions:
-      backport:
-        branches:
-          - version-13-hotfix
-        assignees:
-          - "{{ author }}"
-
   - name: backport to version-14-hotfix
     conditions:
       - label="backport version-14-hotfix"
@@ -61,3 +51,12 @@ pull_request_rules:
         assignees:
           - "{{ author }}"
 
+  - name: backport to version-16-hotfix
+    conditions:
+      - label="backport version-16-hotfix"
+    actions:
+      backport:
+        branches:
+          - version-16-hotfix
+        assignees:
+          - "{{ author }}"