chore: Use GitHub App for update-deps workflow

[lym953]

What does this PR do?

Makes the upgrade workflow use GitHub Apps to get temporary tokens instead of using a long-lived SSH key.

Motivation

This workflow is broken, probably because the SSH key no longer works. Instead of creating a new SSH key, we use a GitHub App, which generates short-lived tokens and is more secure than SSH key.

Testing Guidelines

Will test the workflow after this PR is merged to main because I can't manually trigger it right now.

Additional Notes

Types of Changes

• Bug fix
• New feature
• Breaking change
• Misc (docs, refactoring, dependency upgrade, etc.)

Check all that apply

• This PR's description is comprehensive
• This PR contains breaking changes that are documented in the description
• This PR introduces new APIs or parameters that are documented and unlikely to change in the foreseeable future
• This PR impacts documentation, and it has been updated (or a ticket has been logged)
• This PR's changes are covered by the automated tests
• This PR collects user input/sensitive content into Datadog
• This PR passes the integration tests (ask a Datadog member to run the tests)

Jira: https://datadoghq.atlassian.net/browse/SVLS-6930