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 cncf-maintainer-changes@cncf.io 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.

[krook]

Hi @Rugvip. Question about this change, I see 5 core maintainers in the Backstage owners file, but they don't exactly correspond to the 6 proposed maintainers in this change. Can you please confirm they are correct?

[Rugvip]

@krook yes this is correct still. We recently added more maintainers, but I'd like to get this update through first and then onboard the new ones separately as that'll need invitations etc. This PR just corrects the list as it was quite far off.

[riaankleinhans]

@Rugvip I am looking through projects' maintainer list to make sure we have the right people listed.
I saw the comments above and notices there is still a significant disconnect between the maintainers listed here and in the project repo.
If the project can update the list and send over the mail addresses for all maintainers I can make sure that they are all added to the project maintainers mailing list and service desk

[Rugvip]

@riaankleinhans it's core maintainers + sponsors that should be listed here, I believe those are in sync? I think all core maintainer emails have been shared, but if any of the other ones are missing they're listed in the owners file.