{CI} Migrate fabric bot from portal to config-as-code

[wangzelin007]

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Why migrate fabric bot from portal to config-as-code:

• As fabric bot team are preparing to move permanently to "Config As Code", they have hidden the save button to disable the capability to update the configuration in portal.
• We need to update milestone every month, so we need migrate before 2022-07-05.

What will happen after migration?

• The fabricbot.json file in our repo that will always take precedence than config in portal.
• All update need to commit in .github/fabricbot.json.

[yonzhan]

CI

[wangzelin007]

Successful migration to config-as-code:
#5000