310/captcha_plugins migration changes recaptcha to nogd

The migration assumes that if the existing captcha_plugin config value begins with phpbb_captcha_, then it's one of the standard captchas. The recaptcha plugin is an exception to this convention, however, as it simply uses phpbb_recaptcha. As such, the migrator changes the captcha plugin to nogd, even though recaptcha is still available in 3.1.