Update Backstage maintainers + add back Tremor

[Rugvip]

Checklist for maintainer updates

This updates the Backstage project maintainers to be in sync with the Core Maintainers and Sponsors in backstage/OWNERS.md.

It also restores the Tremors entry that was accidentally removed in #745

• You've provided a link to documentation where the project has approved the maintainer changes.
• The maintainer(s) also created or updated their LFX Individual Dashboard profile.
• You've sent an email with the list of email address(es) to [email protected] for invitations to Service Desk and mailing lists. You can just mark this complete if you are only removing people.
• Optional: You've also sent a PR with affiliation updates to cncf/gitdm.