chore: enable depguard for io/ioutil package

Signed-off-by: Eng Zer Jun <engzerjun@gmail.com>

.golangci.yml

@@ -2,6 +2,13 @@ run:
     timeout: 2m
 
 linters-settings:
+    depguard:
+        list-type: blacklist
+        include-go-root: true
+        packages:
+            - io/ioutil
+        packages-with-error-message:
+            - io/ioutil: "The 'io/ioutil' package is deprecated. Use corresponding 'os' or 'io' functions instead."
     gci:
         local-prefixes: github.com/dexidp/dex
     goimports:
@@ -13,6 +20,7 @@ linters:
     enable:
         - bodyclose
         - deadcode
+        - depguard
         - dogsled
         - exhaustive
         - exportloopref
@@ -64,7 +72,6 @@ linters:
         # - scopelint
 
         # unused
-        # - depguard
         # - goheader
         # - gomodguard