[support-bundle] be explicit about zip compression level

When #8771 integrates we will be setting the compression algorithm but we should also set the compression level.

@smklein comment:

Here's what I would do:

Based on our current set of features, specify the compression level explicitly
Perhaps add a test that "for some arbitrary data, the compression level we picked matches the default"
Then:

If our feature set changes and tweaks the default, the test will fail, and can hopefully identify "what happened".
If the default gets changed, the test will fail, and hopefully identify "what happened".
At that point, we can make the choice of "change our compression level to match the default", or "leave it as-is, to stay stable"

But regardless, I don't think this need to block this PR.

Link to comment: https://github.com/oxidecomputer/omicron/pull/8771#discussion_r2325649693

Start by reviewing PR #8771 and the support-bundle code that selects the zip compression algorithm. Identify where the compression level should be made explicit, add a test using arbitrary data to compare the selected level with the default, and confirm the test documents the intended behavior.