Migrate to actions/create-github-app-token

[leighmcculloch]

What

Replace tibdex/github-app-token@v1 with actions/create-github-app-token@v1 in the CompletedSprint workflow.

Why

The tibdex action is archived and its README directs users to migrate to the official GitHub action.

Close stellar/actions#93

[Copilot]

Pull request overview

This PR migrates from the archived tibdex/github-app-token@v1 action to the official actions/create-github-app-token@v1 action in the CompletedSprint workflow, as the tibdex action is no longer maintained and redirects users to the official GitHub action.

• Updates the GitHub App token generation action to use the official GitHub action
• Maintains the same workflow functionality for updating completed sprint fields on issue closure

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[Copilot]

The official GitHub action actions/create-github-app-token@v1 uses hyphenated parameter names. The input parameters should be app-id and private-key (with hyphens) instead of app_id and private_key (with underscores). The current parameter names were correct for the tibdex action but are incompatible with the official action.

Suggested change

	app_id: ${{ secrets.GH_PROJECT_MANAGEMENT_APP_ID }}
	private_key: ${{ secrets.GH_PROJECT_MANAGEMENT_APP_PEM }}
	app-id: ${{ secrets.GH_PROJECT_MANAGEMENT_APP_ID }}
	private-key: ${{ secrets.GH_PROJECT_MANAGEMENT_APP_PEM }}