dummydevice and musiccast READMEs show the old plugin API

[kalinka-player-bot]

packages/kalinka-plugin-dummydevice/README.md:62-65 and packages/kalinka-plugin-musiccast/README.md:62-65 still show from kalinka_plugin_sdk.api import PluginContext and a module-level def setup(cfg, ctx) -> ExternalOutputDevice. Neither exists any more: plugins subclass OutputDevicePlugin from kalinka_plugin_sdk.plugin and implement async def setup(self, context: OutputDevicePluginContext). Both READMEs were generated from an early version of the template, which this branch has since corrected.

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_{Filed by Kalinka Bot while working on Plugin template pins the SDK to >=2.0,<3 while the SDK is 3.4.}