Allow github.prebuilds.defaultBranch configuration in .gitpod.yml

[jldec]

Let's make defaultBranch synonymous with master.

The currently supported GitHub prebuild configuration settings are (with defaults)

github:
  prebuilds:
    master: true
    branches: false
    pullRequests: true
    pullRequestsFromForks: false
    addCheck: true
    addComment: false
    addBadge: false

See also #7426

[stale[bot]]

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