Removing experimental annotation from JdbcIO

[pabloem]

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[chamikaramj]

LGTM

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[johnjcasey]

@chamikaramj @kennknowles is there any process we should go through with removing an experimental label?

[chamikaramj]

I don't think there's an established process. We can remove experimental tags if we are confident about the stability of the API.

cc: @TheNeuralBit since this API uses Beam Schemas.