Make JSON:API responses easier to compress with gzip, brotli, etc

FWIW it seems CloudFlare supports the current mimetype https://developers.cloudflare.com/speed/optimization/content/brotli/cont...

Unfortunately: JSON:API requires use of the JSON:API media type (application/vnd.api+json) for exchanging data.

So it would be off-spec to use something else. You could discuss alternative options, like go offspec yourself by changing the header with some custom code. But we cannot change how core does this.

I considered changing the header, and it seems to work, but concerned it may lead to issues. We can also configure nginx to compress application/vnd.api+json, but ideally I'd like CloudFront to do it for us.

Happy for this to be closed, but leaving it open for now in case anyone has any novel ideas.