Upgrade mjolnir (v1.6.3 -> v1.6.4)

roles/custom/matrix-bot-mjolnir/defaults/main.yml

@@ -4,7 +4,7 @@
 
 matrix_bot_mjolnir_enabled: true
 
-matrix_bot_mjolnir_version: "v1.6.3"
+matrix_bot_mjolnir_version: "v1.6.4"
 
 matrix_bot_mjolnir_container_image_self_build: false
 matrix_bot_mjolnir_container_image_self_build_repo: "https://github.com/matrix-org/mjolnir.git"