Ref:
- https://github.com/spantaleev/matrix-docker-ansible-deploy/pull/4882
- https://github.com/element-hq/synapse/pull/19204
- https://element-hq.github.io/synapse/latest/usage/configuration/config_documentation.html#enable_local_media_storage

We default it to `true`, keeping up with upstream and the old behavior.

s3-storage-provider users may set `matrix_synapse_enable_local_media_storage` to `false`
to disable local file caching.
This likely comes at the expense of some performance.

For matrix-media-repo users, it likely doesn't matter what this is set to,
as for a matrix-media-repo setup, all media-related API endpoints are
captured and forwarded to matrix-media-repo (before reaching Synapse).