Auto merge of #26785 - servo:dependabot/cargo/gstreamer-0.15.6, r=jdm

build(deps): bump gstreamer from 0.15.5 to 0.15.6

Bumps gstreamer from 0.15.5 to 0.15.6.

[![Dependabot compatibility score](https://api.dependabot.com/badges/compatibility_score?dependency-name=gstreamer&package-manager=cargo&previous-version=0.15.5&new-version=0.15.6)](https://dependabot.com/compatibility-score/?dependency-name=gstreamer&package-manager=cargo&previous-version=0.15.5&new-version=0.15.6)

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[//]: # (dependabot-automerge-start)
[//]: # (dependabot-automerge-end)

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