Replace deprecated ioutil with io and os (#20776)

* Replace ioutil with io and os

* Replace ioutil in utils/file.go

* Minor fix to tests and excluded files

Co-authored-by: Tim Scheuermann <tim.scheuermann@mattermost.com>
Co-authored-by: Mattermod <mattermod@users.noreply.github.com>

api4/saml.go

@@ -5,7 +5,7 @@ package api4
 
 import (
 	"encoding/json"
-	"io/ioutil"
+	"io"
 	"mime"
 	"mime/multipart"
 	"net/http"
@@ -139,7 +139,7 @@ func addSamlIdpCertificate(c *Context, w http.ResponseWriter, r *http.Request) {
 	auditRec.AddMeta("type", d)
 
 	if d == "application/x-pem-file" {
-		body, err := ioutil.ReadAll(r.Body)
+		body, err := io.ReadAll(r.Body)
 		if err != nil {
 			c.Err = model.NewAppError("addSamlIdpCertificate", "api.admin.saml.set_certificate_from_metadata.invalid_body.app_error", nil, err.Error(), http.StatusBadRequest)
 			return