Timestamps improvement; bugfixes

We've made significant improvements to the timestamp accuracy of our Speech-to-Text Best tier for English, Spanish, and German. 96% of timestamps are accurate within 200ms, and 86% of timestamps are now accurate within 100ms.

We've fixed a bug in which confidence scores of transcribed words for the Nano tier would sometimes be outside of the range [0, 1]

We've fixed a rare issue in which the speech for only one channel in a short dual channel file would be transcribed when disfluencies was also enabled.