use default zlib compression level (instead of the maximum), fixes getodk/central#1622

[brontolosone]

Closes getodk/central#1622

I found 3 other places, and thought it best to change it to the default there as well.

What has been done to verify that this works as intended?

CI

Why is this the best possible solution? Were any other approaches considered?

See getodk/central#1622

How does this change affect users? Describe intentional changes to behavior and behavior that could have accidentally been affected by code changes. In other words, what are the regression risks?

In some cases one could end up with a slightly larger ZIP archive. But one would get it faster ;-)

Does this change require updates to the API documentation? If so, please update docs/api.yaml as part of this PR.

N/A

Before submitting this PR, please make sure you have:

• run make test and confirmed all checks still pass OR confirm CircleCI build passes
• verified that any code from external sources are properly credited in comments or that everything is internally sourced