As a Google Voice number user, what app should I use for texting now?
Hangouts still works for texting to and from Google Voice only, and I don't think that is supposed to go away (at least not anytime soon)... it doesn't work as your default SIM carrier's SMS/MMS app anymore. I use it daily and just updated yesterday and it still works with the Google Voice number.

I had the Google Voice app installed, but saw no advantage to it over Hangouts so I continue to use Hangouts and the Hangouts Dialer app and have had no issues so far. That may change later, but for the foreseeable future it is working with Google Voice only. 


  Google Voice users will also be affected, but not as much as
  Hangouts-only users. The rule only applies to messages sent and
  received with your carrier phone number—all SMS messages sent with
  your Google Voice number will remain unaffected. "For SMS users using
  Google Voice on Hangouts on Android Google Voice users who also send
  carrier SMS messages will need to choose another default messaging
  app. Their Google Voice messages will be unaffected and will still be
  available in Google Hangouts," the e-mail states. "For Google Voice
  users on Hangouts on Android Google Voice users who do not use carrier
  SMS, text messaging will not be affected and no notification will be
  shown."
  - Source