Clarify deprecation state of `--silent` option of `rbs validate`

[pocke]

Currently --silent option just displays "`--silent` option is deprecated." . The user cannot decide what they should do from this message. So I updated the message to describe that the silent mode is default and they can use --log-level option also.

ref: https://github.com/ruby/rbs/pull/1648

cc/@ksss