Add SUPPORT_VOLUME_STEP for older Pioneer receivers

[DevSecNinja]

Description:

Older Pioneer models like the Pioneer VSX-528 require SUPPORT_VOLUME_STEP to control the volume.

Doc PR: home-assistant/home-assistant.io#11952

Checklist:

• The code change is tested and works locally.
• Local tests pass with tox. Your PR cannot be merged unless tests pass
• There is no commented out code in this PR.
• I have followed the development checklist

[DevSecNinja]

Thanks for the review @springstan. What is needed to get it merged?

Thanks.

[springstan]

@Cloudenius you could update the docs to include an example for the Pioneer VSX-528 model to show that even these older models work with this integration :)

[DevSecNinja]

@Cloudenius you could update the docs to include an example for the Pioneer VSX-528 model to show that even these older versions work with this integration :)

Done! See doc PR attached. I must say that the telnet connection isn't always reliable and I have to disconnect it from power every couple of weeks to get it to work again. Otherwise it will report that the telnet connection was closed. But the app has the same issues, so it seems like an issue with the device itself.