Move exclude-files to issues section

Tidy up the golangci config file according to the PR discussion in etcd
repo

Reference:
- etcd-io/etcd#17694 (comment)

tools/.golangci.yaml

@@ -1,14 +1,15 @@
 ---
 run:
   timeout: 30m
-  issues.exclude-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