Switch from long-lived tokens to MachineID for the Teleport plugins and provisioning

We are currently using long-lived tokens for the plugins (event handler, Slack) and provisioning via Terraform, which have to be renewed manually once a year. They expires for Teleport Staging last week, and will expire for Teleport Production on 2024-04-10 (next week).

We should switch to MachineID instead so that we no longer have to manage and renew any token ever again:

• https://goteleport.com/docs/machine-id/deployment/gitlab/
• https://goteleport.com/docs/management/export-audit-events/fluentd/#export-an-identity-file-for-the-fluentd-plugin-user
• https://goteleport.com/docs/access-controls/access-request-plugins/ssh-approval-slack/#step-48-export-the-access-plugin-identity