Drop ARMv7 from docker (#9433)

It's proving incredibly hard to build in CircleCI infra.

.circleci/config.yml

@@ -14,7 +14,7 @@ jobs:
           platforms: linux/amd64
       - docker_build:
           tag: -t matrixdotorg/synapse:${CIRCLE_TAG}
-          platforms: linux/amd64,linux/arm/v7,linux/arm64
+          platforms: linux/amd64,linux/arm64
 
   dockerhubuploadlatest:
     docker:
@@ -27,7 +27,7 @@ jobs:
       # until all of the platforms are built.
       - docker_build:
           tag: -t matrixdotorg/synapse:latest
-          platforms: linux/amd64,linux/arm/v7,linux/arm64
+          platforms: linux/amd64,linux/arm64
 
 workflows:
   build:

CHANGES.md

 Synapse 1.xx.0 (2021-xx-xx)
 ===========================
 
+Note that this release drops support for ARMv7 in the official Docker images, due to repeated problems building for ARMv7 (and the associated maintenance burden this entails).
+
 Removal warning
 ---------------
 

changelog.d/9433.docker

+Drop support for ARMv7 in Docker images.