Replace run.skip-files with issues.exclude-files in golangci config

Address the warning "The configuration option `run.skip-files` is
deprecated, please use `issues.exclude-files`."

Signed-off-by: Chun-Hung Tseng <henrybear327@gmail.com>

tools/.golangci.yaml

@@ -1,14 +1,15 @@
 ---
 run:
   timeout: 30m
-  skip-files: [^zz_generated.*]
 issues:
   max-same-issues: 0
   # Excluding configuration per-path, per-linter, per-text and per-source
   exclude-rules:
     # exclude ineffassing linter for generated files for conversion
     - path: conversion\.go
       linters: [ineffassign]
+  exclude-files:
+    - ^zz_generated.*
 linters:
   disable-all: true
   enable: # please keep this alphabetized