Enable gzip compression for files

Why is this change being made?

Our HTML assets aren't being served compressed because our Google Cloud Storage bucket doesn't know that compressing them is okay. In addition, we have a custom Fastly rule that skips caching of HTML files, so Fastly never gets a chance to gzip them either.

We use the -z option (patched with https://github.com/GoogleCloudPlatform/gsutil/pull/1430 via gitlab-org/gitlab-build-images!472 (merged)) to tell Google that certain files can be uploaded compressed. See https://cloud.google.com/storage/docs/gsutil/commands/cp for more details.

Relates to https://gitlab.com/gitlab-com/gl-infra/infrastructure/-/issues/14852

For example:

$ curl -v -s -o /dev/null -H "Accept-Encoding: gzip" https://sh-adding-gzip-encoding.about.gitlab-review.app/index.html |& grep content-
< x-goog-stored-content-encoding: gzip
< x-goog-stored-content-length: 37067
< content-type: text/html
< content-encoding: gzip
< content-length: 37067

Notice content-type is text/html but content-encoding is gzip. Compare this:

$ curl -v -s -o /dev/null -H "Accept-Encoding: gzip" https://about.gitlab.com/ |& grep -i content-
< x-goog-stored-content-encoding: identity
< x-goog-stored-content-length: 151627
< content-type: text/html
< content-length: 151627

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