[HTTP Cache] Validation model: Fix header name
minor #3596 [HTTP Cache] Validation model: Fix header name (bicpi)

This PR was merged into the 2.3 branch.

Discussion
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[HTTP Cache] Validation model: Fix header name

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | all
| Fixed tickets | -

In the cache validation model, the response sends an `ETag` header, but the following requests send an `If-None-Match` header. So the `If-None-Match` header from the request is compared to the `ETag` from the previous response. See `Symfony\Component\HttpFoundation\Request::getETags()` which is used by `Symfony\Component\HttpFoundation\Response::isNotModified()`.

Commits
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f7ef7c0 [HTTP Cache] Validation model: Fix header name