server: drop HTTP/1.1 h2c upgrade handling (#5436)

Use net/http Server.Protocols and the merged golib listener path instead of the deprecated h2c handler. Keep HTTP/1.1 and cleartext HTTP/2 prior-knowledge support, while intentionally dropping HTTP/1.1 Upgrade: h2c.

.golangci.yml

@@ -61,11 +61,6 @@ linters:
     - legacy
     - std-error-handling
     rules:
-    # Keep h2c and HTTP/1.1 Upgrade:h2c compatibility until an explicit migration.
-    - linters:
-      - staticcheck
-      path: ^pkg/util/vhost/http\.go$
-      text: "^SA1019:"
     - linters:
       - errcheck
       - maligned