Add support for Ubuntu 26.10 'Stonking Stingray' (#20428)

Fix https://github.com/element-hq/synapse/issues/20040

Associated PR: https://github.com/matrix-org/pkg-repo-configs/pull/32

This PR was written following the instructions in Element's internal
handbook page for [adding a new
distribution](https://handbook.element.io/books/backend-team/page/synapse-debian-packages#bkmrk-adding-a-new-distrib).

changelog.d/20428.misc

@@ -0,0 +1 @@
+Add package build targets for Ubuntu 26.10 'Stonking Stingray'.

scripts-dev/build_debian_packages.py

@@ -32,6 +32,7 @@ DISTS = (
     "ubuntu:noble",  # 24.04 LTS (EOL 2029-05-31) (our EOL forced by Python 3.12 is 2028-10-31)
     "debian:trixie",  # (EOL 2030-06-30) (our EOL forced by Python 3.13 is 2029-10-31)
     "ubuntu:resolute",  # 26.04 (EOL 2031-04-30) (our EOL forced by Python 3.14 is 2030-10-31)
+    "ubuntu:stonking",  # 26.10 (EOL 2027-07-15) (our EOL forced by Python 3.14 is 2030-10-31)
 )
 
 DESC = """\