Cache Deck assets differently

[xrstf]

This PR changes the client-side caching of static Deck assets.

There was already #6395, which added the original client-side caching code. It's not super obvious from the code, but this method relied on the Last-Modified header and the browser sending an If-Not-Modified header on subsequent revalidations. This works just fine when you run Deck locally, but if you peek into the container image for Deck, you'll see

I presume this is on purpose to achieve reproducible container images.

The Go HTTP serve will not send a Last-Modified header for files that have a 0 timestamp. Which is the case for all of them 😁 This unfortunately totally defeats the caching in a production setup.

Later #10626 introduced the deckVersion, but so far only used it to display it in the sidebar.

I now make use of this variable and add it as a cache busting header. This allows Deck to send strong caching headers, while after a Prow update clients would still be forced to fetch the new versions of all assets. This works independently of any file timestamps and I think is more robust in general.

Since #6395 was authored 6 years ago, I decided to also drop IE 6 support. At this point IE 6 is truly dead and MDN recommends not setting the Pragma header anymore. However I kept the HTTP/1.0-only proxy support, as I can imagine some complex Prow setups with legacy proxies in-between that are hard to upgrade.

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Hi @xrstf. Thanks for your PR.

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[xmudrii]

/ok-to-test

[xmudrii]

/retest

[xrstf]

/retest

[michelle192837]

Looks good, thanks!

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: michelle192837, xrstf

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