Change X-Debug plugin's x-cache order

[mlibbey]

In #6328 we document the X-Debug plugin's behavior with multiple proxies. Unfortunately, its done in the opposite order of the Via header (which is append, based on https://tools.ietf.org/html/rfc7230#section-5.7.1). This is confusing -- we can have 2 headers next to each other with similar data, but, needs to be read in the exact opposite order.

Proposal is to standardize the X-Cache header to behave like Via.

[github-actions]

This issue has been automatically marked as stale because it has not had recent activity. Marking it stale to flag it for further consideration by the community.

[randall]

Fixed by #6333