ci: upstream pipeline runs only as a cron job

and no longer runs on pull requests and pushes.

Closes #2449

.github/workflows/ci.yml

@@ -234,7 +234,7 @@ jobs:
       fail-fast: false
       matrix:
         sys: ["enable", "disable"]
-        ruby: ["2.6", "2.7", "3.0", "3.1", "mingw", "head"]
+        ruby: ["2.6", "2.7", "3.0", "3.1", "mingw"]
     runs-on: windows-2022
     steps:
       - name: configure git crlf

.github/workflows/upstream.yml

@@ -4,16 +4,6 @@ concurrency:
   cancel-in-progress: true
 on:
   workflow_dispatch:
-  push:
-    branches:
-      - main
-      - v*.*.x
-    tags:
-      - v*.*.*
-  pull_request:
-    types: [opened, synchronize]
-    branches:
-      - '*'
   schedule:
     - cron: "0 8 * * 1,3,5" # At 08:00 on Monday, Wednesday, and Friday # https://crontab.guru/#0_8_*_*_1,3,5