tiup plugin config is changed

[aytrack]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

1. deploy a v5.4.1 cluster with plugin, which config like

plugin.dir: "path/to/plugin"
plugin.load: "whitelist-1,audit-1"

2. upgrade this cluster to v6.1.0 without edit the config
3. check the plugins status show plugins

2. What did you expect to see? (Required)

1. after upgrade, the plugin will work well

3. What did you see instead (Required)

1. the plugin not load

#33279 changed the config to instance. you need to change the config to

instance.plugin_dir: "path/to/plugin"
instance.plugin_load: "audit-1,whitelist-1"

4. What is your TiDB version? (Required)

mysql root@172.16.4.131:(none)> select tidb_version()\G
***************************[ 1. row ]***************************
tidb_version() | Release Version: v6.1.0
Edition: Community
Git Commit Hash: f87623664f5417497cc828e03079762bee8b0b5b
Git Branch: heads/refs/tags/v6.1.0
UTC Build Time: 2022-05-23 10:12:19
GoVersion: go1.18.2
Race Enabled: false
TiKV Min Version: v3.0.0-60965b006877ca7234adaced7890d7b029ed1306
Check Table Before Drop: false