Lower gzip compression threshold

Lower the minimum size required for nginx to compress responses.

• Reduce the number of requests that need more than one packet. For example, small json respones, realtime_changes respones, small raw blobs.
• For gitlab.com, our CDN compresses requests of this size.

https://gitlab.com/gitlab-com/gl-infra/infrastructure/-/issues/10073

Signed-off-by: Ben Kochie bjk@gitlab.com