Upgrade Dendrite (0.8.1 -> 0.9.0)

Looks like we've skipped a bunch of 0.8.x versions (up to 0.8.9)
and are jumping straight to 0.9.0.

This is untested. Judging by Dendrite's changelog, it shouldn't cause
any breakage though: https://github.com/matrix-org/dendrite/blob/v0.9.0/CHANGES.md

roles/matrix-dendrite/defaults/main.yml

@@ -6,7 +6,7 @@ matrix_dendrite_enabled: true
 
 matrix_dendrite_docker_image: "{{ matrix_dendrite_docker_image_name_prefix }}matrixdotorg/dendrite-monolith:{{ matrix_dendrite_docker_image_tag }}"
 matrix_dendrite_docker_image_name_prefix: "docker.io/"
-matrix_dendrite_docker_image_tag: "v0.8.1"
+matrix_dendrite_docker_image_tag: "v0.9.0"
 matrix_dendrite_docker_image_force_pull: "{{ matrix_dendrite_docker_image.endswith(':latest') }}"
 
 matrix_dendrite_base_path: "{{ matrix_base_data_path }}/dendrite"