client-api: Update MSC3814 dehydrated device events to GET

[livenson]

MSC3814 now fetches the events of a dehydrated device with GET .../dehydrated_device/{device_id}/events, with the batch token in the from query parameter. It also leaves next_batch out of the response once there are no more events. The MSC commits are matrix-org/matrix-spec-proposals@72c06568 and @9ea19e2f, and Synapse implements them in element-hq/synapse#19896 and #19897.

This updates get_events::unstable to that definition:

• The request is GET with #[ruma_api(query)] from.
• The response's next_batch is skip_serializing_if = "Option::is_none".

The previous POST definition moves unchanged to get_events::unstable_v1, so a homeserver can keep serving clients that still use it, such as matrix-rust-sdk. This follows how update_delayed_event::unstable_v1 kept the earlier MSC4140 shape.

Why it matters: matrix-js-sdk 42 and later uses the GET. A homeserver that only routes the POST answers it with 405. The client then replaces its dehydrated device, and the room keys queued for that device are lost. A "next_batch": null on the last page also makes those clients restart paging from the first page indefinitely.

The companion Tuwunel change, which serves both methods, is matrix-construct/tuwunel#646.

Checks:

• Nightly cargo fmt is clean.
• cargo clippy -p ruma-client-api --features client,server,unstable-msc3814 -- -D warnings is clean.
• The dehydrated-device tests pass, including the three new ones: GET request serialization with from, server-side query parsing, and a last page without next_batch.