Add Ubuntu 26.10 'Stonking Stingray' package support (action in 2026-10)

This is a tracking issue to add Ubuntu 26.10 'Stonking Stingray' package support which arrives in 2026-10 (October).

### Dev notes

See handbook docs: https://handbook.element.io/books/backend-team/page/synapse-debian-packages#bkmrk-adding-a-new-distrib

### Todo

- [ ] Update list of supported dists in [`scripts-dev/build_debian_packages.py#L29-L36`](https://github.com/element-hq/synapse/blob/415a869f1f53bd4bf22a69e74081875dc9c17735/scripts-dev/build_debian_packages.py#L29-L36)
- [ ] Update https://github.com/matrix-org/pkg-repo-configs ([example](https://github.com/matrix-org/pkg-repo-configs/pull/31))

Read the handbook section on adding a new distribution, then inspect scripts-dev/build_debian_packages.py lines 29-36. Update the supported distribution list and review the example change in pkg-repo-configs; done means both repositories support Ubuntu 26.10 package builds.

Written by the indexing model from the issue text.