"history_visibility: shared" is incompatible with encryption

[richvdh]

When you join an encrypted room, there is no way to access historical encrypted content. The "shared" (and "world_readable") history visibility settings are therefore somewhat misleading.

• "Members only (since the point in time of selecting this option)" makes no sense for encrypted rooms element-web#27977
• Element X doesn't currently expose the setting to change history visibility, so the problem is less acute. However, we may still consider changing the default history visibility for new rooms.