Use Colab `serve_kernel_port_as_iframe` API

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The Colab folks have upstreamed our iframe creation logic with a helper
modeled after ours:

https://github.com/googlecolab/colabtools/blob/f31c2c03993b733b8f5ae7c7a451077f72ca6882/google/colab/output/_util.py#L66-L78

It’s available as:

from google.colab import output
output.serve_kernel_port_as_iframe(port)

…and we already only render to Colab if google.colab is present.
May as well use it, simplifying our code a bit?