Update README.md to mention EZSP v9

[Hedda]

Update README.md to mention EZSP v9 Protocol Version as default in Silabs Zigbee EmberZNet 7.1.x firmware.

Silicon Labs Gecko SDK 4.1.0 (GSDK 4.1) was released a couple of days ago and with it Zigbee EmberZNet SDK 7.1.0.0 and tube0013 submitted patch with initial support in PR https://github.com/zigpy/bellows/pull/462

https://github.com/SiliconLabs/gecko_sdk/releases

https://www.silabs.com/documents/public/release-notes/emberznet-release-notes-7.1.0.0.pdf

https://web.archive.org/web/20220401230237/https://www.silabs.com/documents/public/user-guides/ug100-ezsp-reference-guide.pdf

https://www.silabs.com/documents/public/user-guides/ug100-ezsp-reference-guide.pdf