ci: fix linter warnings for preallocation

Signed-off-by: Jan Larwig <jan@larwig.com>

pkg/validation/allowlist.go

@@ -11,6 +11,7 @@ import (
 )
 
 func validateAllowlists(o *options.Options) []string {
+	//nolint:prealloc
 	msgs := []string{}
 
 	msgs = append(msgs, validateAuthRoutes(o)...)

pkg/validation/header.go

@@ -7,6 +7,7 @@ import (
 )
 
 func validateHeaders(headers []options.Header) []string {
+	//nolint:prealloc
 	msgs := []string{}
 	names := make(map[string]struct{})
 

pkg/validation/upstreams.go

@@ -9,6 +9,7 @@ import (
 )
 
 func validateUpstreams(upstreams options.UpstreamConfig) []string {
+	//nolint:prealloc
 	msgs := []string{}
 	ids := make(map[string]struct{})
 	paths := make(map[string]struct{})