ci: update ci config to consider v15 (#22822)

.github/workflows/initiate_release.yml

@@ -15,7 +15,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        version: ["13", "14"]
+        version: ["13", "14", "15"]
 
     steps:
       - uses: octokit/request-action@v2.x

.github/workflows/patch-mariadb-tests.yml

@@ -134,6 +134,7 @@ jobs:
           }
 
           update_to_version 14
+          update_to_version 15
 
           echo "Updating to last commit"
           pgrep honcho | xargs kill

.mergify.yml

@@ -9,6 +9,7 @@ pull_request_rules:
           - author!=frappe-pr-bot
           - author!=mergify[bot]
         - or:
+          - base=version-16
           - base=version-15
           - base=version-14
           - base=version-13
@@ -71,3 +72,13 @@ pull_request_rules:
         assignees:
           - "{{ author }}"
 
+  - name: backport to version-15-hotfix
+    conditions:
+      - label="backport version-15-hotfix"
+    actions:
+      backport:
+        branches:
+          - version-15-hotfix
+        assignees:
+          - "{{ author }}"
+