Synapse 1.126.0rc2 (2025-03-05)

Administrators using the Debian/Ubuntu packages from packages.matrix.org, please check
the relevant section in the upgrade notes
as we have recently updated the expiry date on the repository's GPG signing key. The old version of the key will expire on 2025-03-15.

Internal Changes

• Fix wheel building configuration in CI by installing libatomic1. (#18212, #18213)