[treescript] Stop using the `releng-treescript` app for Gecko l10n bumps

We added the ability to store l10n changes in a separate Github repo for Gecko l10n bumps:
https://github.com/mozilla-releng/scriptworker-scripts/commit/3938c24da2c0964a1adc9994c3a8300df65ee0ae

We currently authenticate as the releng-treescript app to do things like get the latest commit and return the contents of files. This is useful for avoiding Github rate limits. However, we do not actually commit any changes to these external l10n repos. Therefore using releng-treescript, which has write access to a repo, does not follow the Principle of Least Privilege.

Instead, I believe we should create a generic releng-readonly (name TBD) app that only has read access to the repos. Then, we can install this app anywhere we simply want read access (e.g for higher rate limits or for access to private repos).

Comienza revisando el commit referenciado y la autenticación actual utilizada para las actualizaciones de l10n de Gecko y los repositorios externos de l10n. Determina dónde se necesita acceso de solo lectura sin acceso para hacer commits. Se considera terminado cuando el acceso de solo lectura a los repositorios utiliza una aplicación separada en lugar de releng-treescript, y se han resuelto el nombre de la aplicación y el alcance de su instalación.