Adopt the modbus-connection library (and help test it)

We're adopting Home Assistant's modbus-connection library for our v4.0.0 release.

The first release will focus on minimal changes to adopt the library and to remove unsupported features from the integration without substantially altering the core code so we're not doing a full rewrite in one step (which has a higher chance of introducing regressions). The first changes reduce integration code by about 15%. Future releases will explore leveraging more features provided by modbus-connection.

Feedback for the v4.0.0 pre-releases can be added to this issue or to the Modernizing Modbus in Home Assistant discussion.