Consolidate bug labels (#24707)

Summary:
We currently have "Type: Bug Report", "Type: Bug Fix", and "Impact: Bug" in our list of labels. To simplify life for maintainers, lets use a single Bug label.

This PR makes it so new Bug Reports use the "Bug" label. The bot has been updated to accept "Bug" as a valid template label already.

[General] [Changed] - New issues use the Bug label
Pull Request resolved: https://github.com/facebook/react-native/pull/24707

Differential Revision: D15220772

Pulled By: hramos

fbshipit-source-id: e25f7180839d2e6aa7211b805dfbf5368b39d738