Deprecate and remove `user_segment`

ref https://github.com/getsentry/team-sdks/issues/44 and https://github.com/getsentry/sentry-ruby/pull/2493

As per https://github.com/getsentry/sentry/issues/58767#issuecomment-1808116241, user segment is deprecated from the user interface and the SDKs. We should work toward removing it in the Ruby SDK.