Hide device displaynames from other users altogether, even on the same homeserver

This issue has been migrated from [#14309](https://github.com/matrix-org/synapse/issues/14309).

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Other users on my homeserver don't really need to know the [displayname](https://spec.matrix.org/v1.4/client-server-api/#put_matrixclientv3devicesdeviceid) of my devices, especially since the default of some clients is to share the client name and operating system identifiers.

I propose that we forgo a config option (unlike [`allow_device_name_lookup_over_federation`](https://matrix-org.github.io/synapse/latest/usage/configuration/config_documentation.html#allow_device_name_lookup_over_federation)) and simply prevent local users from seeing the device displaynames of other local users by default. (I'm open to feedback on use cases where a config option would be necessary.)

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Note that sytest currently assumes that device displaynames are always shared between local users, so [this test](https://github.com/matrix-org/sytest/blob/4784b7ed3337962e7938d368d2748953d627d5bf/tests/50federation/40devicelists.pl#L190) will need to be updated.