Replace hardcoded AppProperties values with Gradle ${} placeholders,
allowing version/channel/vendor to be configured via gradle.properties
or -P flags at build time.
Also refactor webhook configuration to flatten the properties hierarchy
by removing the intermediate WebhookProperties wrapper.
Replace GitLabWebhookProperties with GitHubWebhookProperties (secret, allowed-users).
Update WebhookProperties and WebhookManager to use GitHub-only configuration.
Add app.webhook.github defaults to application.yaml.
siujamo
zihluwang
