Renew `packages.matrix.org` Debian signing key (expires on 2027-03-15) (action on 2026-08-03)

The public signing key for the Synapse Debian package repository (`packages.matrix.org`) expires on 2027-03-15. The expiry date is kept short (2-3 years) in case the private key is lost or stolen. We should aim to update the expiry date 2-3 months in advance (minimum) of it expiring in order to give users time to update their local copy of the key. Otherwise, after it expires, they won't be able to validate updates to their Synapse Debian package until they update the key. Per the [weekly backend sync meeting](https://docs.google.com/document/d/1ik0aDdzpNhIoByACU8kXxNGqgqkjPMTSPTrydpOd6-E/edit?tab=t.0#bookmark=id.t1prpv9srr44), we're good to start this process now (better earlier than later) so the action date for this task is today (2026-08-03). ### Documentation We have [documentation](https://handbook.element.io/books/backend-team/page/synapse-debian-packages#bkmrk-renewing-the-debian-) on how to update the signing keys. ~~From those docs, it's unclear to me whether everyone just has as copy of the master key, and how those are distributed (everyone has to access the safe to get a copy?)~~ There is a private master signing key in the safe and then each developer who signs things has private subkey derived from the master key. Previous expiration 2023: https://github.com/matrix-org/synapse/issues/10389#issuecomment-1472581997 Previous expiration 2025: https://matrix.to/#/!SGNQGPGUwtcPBUotTL:matrix.org/$uspckOk5hoN_o-RytjqqnPAZE6CgKLF8Zhv7nQjkJYw?via=jki.re&via=element.io&via=matrix.org ### Todo - [x] Consider whether private master signing key also needs updating (expires in 2033 (TODO: exact date)) - Action date: 2026-08-03 - Not necessary because (assumed to be years out still) - [x] Update the expiry of the public signing keys - Action date: 2026-08-03 - [x] Nothing is needed for developers with Yubikeys that already have private signing keys (subkey of the master key). Updating the expiry is part of the public key and the private keys remain the same - [x] Publish the updated public keys - Action date: 2026-08-03 - [x] Let those who use the downstream Synapse Debian packages (the community, our customers, package maintainers room) know in the next Synapse release that they should expect to update the `matrix-org-archive-keyring` package before installing the latest Synapse release. - Action date: 2026-08-05 (now that public keys have been published) - [x] Add to Synapse upgrade notes, https://github.com/element-hq/synapse/pull/20066 - [x] Announce alongside next Synapse release/RC -> [RC announcement](https://matrix.to/#/!i5D5LLct_DYG-4hQprLzrxdbZ580U9UB6AEgFnk6rZQ/$D5CAfrYLOFpXUehGfOse4YM9A3losrGtIzW5w71M0MU?via=element.io&via=matrix.org&via=beeper.com)