update changelog

Signed-off-by: Ben Ye <[email protected]>

CHANGELOG.md

@@ -45,6 +45,7 @@ We use *breaking :warning:* to mark changes that are not backward compatible (re
 - [#6325](https://github.com/thanos-io/thanos/pull/6325) Store: return gRPC resource exhausted error for byte limiter.
 - [#6399](https://github.com/thanos-io/thanos/pull/6399) *: Fix double-counting bug in http_request_duration metric
 - [#6428](https://github.com/thanos-io/thanos/pull/6428) Report gRPC connnection errors in the logs.
+- [#6509](https://github.com/thanos-io/thanos/pull/6509) Store Gateway: Remove `memWriter` from `fileWriter` to reduce memory usage when sync index headers.
 
 ### Changed
 - [#6049](https://github.com/thanos-io/thanos/pull/6049) Compact: *breaking :warning:* Replace group with resolution in compact metrics to avoid cardinality explosion on compact metrics for large numbers of groups.