Move Kotlin library to its own repo (firechip/cobs_codec_kt)

The pure-Kotlin Android COBS library now lives standalone at https://github.com/firechip/cobs_codec_kt with its own releases and CI. Removes the duplicated cobs_codec_kt/ subproject, the release-aar workflow, and the .pubignore entry that excluded it. This keeps this repository focused on the Dart package.

Signed-off-by: Alexander Salas Bastidas <ajsb85@firechip.dev>

Author: ajsb85