storage: rename term -> last_log_term in TimelineMembershipSwitchResponse (#12481)

## Problem
Names are not consistent between safekeeper migration RFC and the actual
implementation.

It's not used anywhere in production yet, so it's safe to rename. We
don't need to worry about backward compatibility.

- Follow up on https://github.com/neondatabase/neon/pull/12432

## Summary of changes
- rename term -> last_log_term in TimelineMembershipSwitchResponse 
- add missing fields to TimelineMembershipSwitchResponse in python

libs/safekeeper_api/src/models.rs

@@ -221,7 +221,7 @@ pub struct TimelineMembershipSwitchRequest {
 pub struct TimelineMembershipSwitchResponse {
     pub previous_conf: Configuration,
     pub current_conf: Configuration,
-    pub term: Term,
+    pub last_log_term: Term,
     pub flush_lsn: Lsn,
 }