az functionapp should not set `FUNCTIONS_EXTENSION_VERSION` for function apps with custom docker images

[Francisco-Gamino]

az feedback auto-generates most of the information requested below, as of CLI version 2.0.62

Describe the bug
az function app should not set FUNCTIONS_EXTENSION_VERSION for function apps with custom docker images

This is the line of code that sets the function app setting
https://github.com/Azure/azure-cli/blob/dev/src/azure-cli/azure/cli/command_modules/appservice/custom.py#L2434

[Francisco-Gamino]

cc: @gzuber @pragnagopa @ankitkumarr

Thanks for the feedback! We are routing this to the appropriate team for follow-up. cc @ahmedelnably, @fabiocav

[yonzhan]

functionapp