automation: Update matrix-synapse Docker tag to v1.73.0

This MR contains the following updates:

Package	Update	Change
matrix-synapse	minor	1.72.0 -> 1.73.0

---

Release Notes

matrix-org/synapse

v1.73.0

Compare Source

===========================

Please note that legacy Prometheus metric names have been removed in this release; see the upgrade notes for more details.

No significant changes since 1.73.0rc2.

---

Configuration

📅 Schedule: Branch creation - At any time (no schedule defined), Automerge - At any time (no schedule defined).

🚦 Automerge: Disabled by config. Please merge this manually once you are satisfied.

♻ Rebasing: Whenever MR becomes conflicted, or you tick the rebase/retry checkbox.

🔕 Ignore: Close this MR and you won't be reminded about this update again.

---

• If you want to rebase/retry this MR, check this box

---

This MR has been generated by Renovate Bot. The local configuration can be found in the local Renovate Bot repository.