fix: Update app.json, Heroku deployment is broken (appsmithorg#19540)

## Description

After Heroku removes the free tier for their service, app.json is now
outdated.

Fixes appsmithorg#19538

## Type of change

- Bug fix (non-breaking change which fixes an issue)

## How Has This Been Tested?

- Manual , Heroku install

## Checklist:
### Dev activity
- [x] My code follows the style guidelines of this project
- [x] I have performed a self-review of my own code
- [x] I have commented my code, particularly in hard-to-understand areas
- [x] I have made corresponding changes to the documentation
- [x] My changes generate no new warnings
- [x] I have added tests that prove my fix is effective or that my
feature works
- [x] New and existing unit tests pass locally with my changes
- [ ] PR is being merged under a feature flag


### QA activity:
- [ ] Test plan has been approved by relevant developers
- [ ] Test plan has been peer reviewed by QA
- [ ] Cypress test cases have been added and approved by either SDET or
manual QA
- [ ] Organized project review call with relevant stakeholders after
Round 1/2 of QA
- [ ] Added Test Plan Approved label after reveiwing all Cypress test

app.json

@@ -8,7 +8,7 @@
     ],
     "addons": [
         {
-            "plan": "heroku-redis:hobby-dev",
+            "plan": "heroku-redis:mini",
             "as": "APPSMITH_REDIS"
         }
     ],