net-im/py-matrix-synapse: Update to 1.56.0

• Mention important changes related to the upgrade to UPDATING
• Update the dependency constraints

Co-authored-by: Sascha Biberhofer <ports at skyforge dot at>
PR: 263193
Approved by: Sascha Biberhofer <ports at skyforge dot at> (maintainer)

(cherry picked from commit 76ad2b55b71f9ba18a205d9ecdb248b78d626a0c)