Increase 500 max connection limit for Heroku Postgres

### Required Terms

- [X] I agree to follow this project's [Code of Conduct](https://github.com/heroku/roadmap/blob/main/CODE_OF_CONDUCT.md)
- [X] I have read and accept the [Salesforce Program Agreement](https://www.salesforce.com/company/program-agreement/)

### What service(s) is this request for?

Heroku Postgres

### Tell us about what you're trying to solve. What challenges are you facing?

Currently, the largest Heroku Postgres plans support a maximum of 500 connections. This value dates from 2013 and was a careful tradeoff between performance, parallelism and resource use. Here are some related write-ups:

* https://blog.heroku.com/connection_limit_guidance
* https://brandur.org/postgres-connections

Heroku also supports both [client-side](https://devcenter.heroku.com/articles/best-practices-pgbouncer-configuration#configuring-the-client-side-buildpack) and [server-side](https://devcenter.heroku.com/articles/postgres-connection-pooling) to manage connections.

Some customers may, however, still want more than 500 connections and newer Postgres versions may better support higher connection counts. This issue is for us to explore offering higher connection counts.