Use strings.SplitSeq for Cache-Control and Vary parsing

golangci-lint's modernize analyzer flags ranging over strings.Split;
strings.SplitSeq avoids allocating the intermediate slice.

Refs #3025

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
Copilot-Session: 4959e1f9-f8e6-4e97-a487-f395a0123c79