fix: set gzip level to 6 for preview server

[chenjiahan]

Summary

In the previous PR (#3090), I changed the default gzip level to 1 for better performance. But for the preview server, it is better to use level: 6 because it can simulate the common gzip compression rates and reflects a more realistic file size.

Related Links

https://github.com/madler/zlib/blob/ac8f12c97d1afd9bafa9c710f827d40a407d3266/zlib.h#L239

Checklist

• Tests updated (or not required).
• Documentation updated (or not required).

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