Rename the key field of the SecretStorageDefaultKeyEventContent to key_id

This makes it clearer that the field does not contain a secret key of
any sort.

crates/ruma-events/CHANGELOG.md

@@ -9,6 +9,8 @@ Bug fixes:
 
 Breaking changes:
 
+- Rename the `key` field in the `SecretStorageDefaultKeyEventContent` to
+  `key_id`.
 - Make `in_reply_to` field of `Thread` optional
   - It was wrong to be mandatory, spec was unclear (clarified [here](https://github.com/matrix-org/matrix-spec/pull/1439))
 - Remove `AnswerSessionDescription` and `OfferSessionDescription` types, use `SessionDescription`

crates/ruma-events/src/secret_storage/default_key.rs

@@ -12,5 +12,6 @@ use crate::macros::EventContent;
 #[ruma_event(type = "m.secret_storage.default_key", kind = GlobalAccountData)]
 pub struct SecretStorageDefaultKeyEventContent {
     /// The ID of the default key.
-    pub key: String,
+    #[serde(rename = "key")]
+    pub key_id: String,
 }