Show warning log when a deprecated config is used

[t-kikuc]

What would you like to be added:

Show warning log like when loading deprecated configs [WARN Deprecated] 'Slack' is deprecated. Please use 'SlackUsers'/'SlackGroups' instead. See xxx for details..

deprecated fields (any other?):

• EventWatcher(.pipe/)
• NotificationMention.slack
• CustomSyncStageOptions

cf. log like this:

$ make run/site
env RELEASE=v0.48.4 hugo server --source=docs
WARN  DEPRECATED: Kind "taxonomyterm" used in disableKinds is deprecated, use "taxonomy" instead.
ERROR deprecated: config: languages.en.description: custom params on the language top level was deprecated in Hugo v0.112.0 and will be removed in Hugo 0.129.0. Put the value below [languages.en.params]. See https://gohugo.io/content-management/multilingual/#c

Why is this needed:

To inform users that they should migrate before removing deprecated code.