Remove usage of ADMIN_GITHUB_TOKEN for releasing with the Jupyter Releaser

[jtpio]

The Jupyter Releaser updated its example workflows to use a GitHub App instead of relying on the ADMIN_GITHUB_TOKEN: https://github.com/jupyter-server/jupyter_releaser/blob/89be0fbdc13f47011c95a870218c18d94c4f193b/example-workflows/publish-release.yml#L24

For reference this is already in use in some other Jupyter projects, for example Jupyter Server: https://github.com/jupyter-server/jupyter_server/blob/69361eeb2702572faca6e6cc8ac1003e393eb5aa/.github/workflows/publish-release.yml#L24

We could start doing the same for the repos under the voila-dashboards organization, following the guide: https://jupyter-releaser.readthedocs.io/en/latest/how_to_guides/convert_repo_from_repo.html#checklist-for-adoption

[jtpio]

Opened PRs on several repos currently using the releaser:

• Update releaser workflows #1453
• Update releaser workflows voila-gridstack#223
• Update releaser workflows voici#114

@martinRenou @trungleduc are there any other repos in the voila-dashboards org using the releaser?

[trungleduc]

Thanks @jtpio for working on it, https://github.com/voila-dashboards/voila-topbar still uses the admin token

[jtpio]

OK thanks, just opened voila-dashboards/voila-topbar#11

[jtpio]

ADMIN_GITHUB_TOKEN has now been removed from the org secrets.

The new credentials for the GitHub app have been added to the Jupyter 1Password (cc @martinRenou as SSC rep for Voila)