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

test_runner/fixtures/safekeeper/http.py

@@ -112,12 +112,18 @@ class TimelineCreateRequest:
 class TimelineMembershipSwitchResponse:
     previous_conf: MembershipConfiguration
     current_conf: MembershipConfiguration
+    last_log_term: int
+    flush_lsn: Lsn
 
     @classmethod
     def from_json(cls, d: dict[str, Any]) -> TimelineMembershipSwitchResponse:
         previous_conf = MembershipConfiguration.from_json(d["previous_conf"])
         current_conf = MembershipConfiguration.from_json(d["current_conf"])
-        return TimelineMembershipSwitchResponse(previous_conf, current_conf)
+        last_log_term = d["last_log_term"]
+        flush_lsn = Lsn(d["flush_lsn"])
+        return TimelineMembershipSwitchResponse(
+            previous_conf, current_conf, last_log_term, flush_lsn
+        )
 
 
 class SafekeeperHttpClient(requests.Session, MetricsGetter):