configure PARSE_SERVER_CLEANUP_INVALID_INSTALLATIONS without environment vars

[mortenmo]

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Issue Description

Most configuations can be done through options to Parse Server, but PARSE_SERVER_CLEANUP_INVALID_INSTALLATIONS doesn't seem to be supported that way.

Suggest to add this config to the Parse Server options, for people who uses push through Parse this is a very important thing to turn on so Apple doesn't block for sending too many repeats to disabled devices.

Environment

Server

• Parse Server version: 7.0.0
• Operating system: OSX
• Local or remote host (AWS, Azure, Google Cloud, Heroku, Digital Ocean, etc): Local

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[mtrezza]

The feature is still flagged as experimental, see #6052. We actually do not support the concept of "experimental features" anymore, so let's clean up this clean up flag 🙂

The documentation for this option must incorporate the caveats described in #6052. The feature in fact still has its shortcomings and risks.