lint: enable canonicalheader linter (#7330)

Enable canonicalheader linter to enforce proper HTTP header casing.
This ensures headers use Go's canonical format (e.g., "Content-Type"
instead of "content-type") for consistency.

Fixes header casing in DoH implementation.

Signed-off-by: Ville Vesilehto <ville@vesilehto.fi>

.golangci.yml

@@ -2,6 +2,7 @@ version: "2"
 linters:
   default: none
   enable:
+    - canonicalheader
     - copyloopvar
     - govet
     - ineffassign