chore(heroku): Switch heroku stack to the latest stack heroku-24 (#10085)

# Pull Request Template

## Description


- Heroku has marked `heroku-20` stack as EOL by April 2025
- https://help.heroku.com/NPN275RK/heroku-20-end-of-life-faq 
- https://devcenter.heroku.com/articles/heroku-24-stack

Fixes https://linear.app/chatwoot/issue/CW-3552/upgrade-heroku-stack-to-heroku-24

app.json

@@ -48,7 +48,7 @@
       "size": "basic"
     }
   },
-  "stack": "heroku-20",
+  "stack": "heroku-24",
   "image": "heroku/ruby",
   "addons": [
     {
@@ -58,7 +58,7 @@
       "plan": "heroku-postgresql:essential-0"
     }
   ],
-  "stack": "heroku-20",
+  "stack": "heroku-24",
   "buildpacks": [
     {
       "url": "heroku/nodejs"