upgrade to heroku stack 24 with nginx

[chavakula]

Description

updating heroku stack 20 to stack 24 due to deprecation of stack 20 by heroku on april 30th 2025

Motivation and Context

moved to nginx based buildpack, static buildpack is not supported in stack 24

Review Instructions

[chavakula]

Steps followed :

Prerequeste
use "heroku login" to login to heroku cli and then continue with below steps

1. heroku buildpacks:clear -a sensu-docs-stage
2. heroku buildpacks:set heroku/nodejs -a sensu-docs-stage
3. heroku buildpacks:add --app sensu-docs-stage heroku-community/nginx

Above steps will clear any buildpacks first, then it will set nodejs as buildpack and then finally add nginx support to serve the requests.