### ClickHouse release v21.6.7.57-stable FIXME as compared to v21.6.6.51-stable

#### Bug Fix
* Backported in [#25955](https://github.com/ClickHouse/ClickHouse/issues/25955): Fix `ALTER MODIFY COLUMN` of columns, which participates in TTL expressions. [#25554](https://github.com/ClickHouse/ClickHouse/pull/25554) ([Anton Popov](https://github.com/CurtizJ)).
* Backported in [#25959](https://github.com/ClickHouse/ClickHouse/issues/25959): Fix extremely long backoff for background tasks when the background pool is full. Fixes [#25836](https://github.com/ClickHouse/ClickHouse/issues/25836). [#25893](https://github.com/ClickHouse/ClickHouse/pull/25893) ([alesapin](https://github.com/alesapin)).
* Backported in [#26096](https://github.com/ClickHouse/ClickHouse/issues/26096): Fix wrong thread estimation for right subquery join in some cases. Close [#24075](https://github.com/ClickHouse/ClickHouse/issues/24075). [#26052](https://github.com/ClickHouse/ClickHouse/pull/26052) ([Vladimir C](https://github.com/vdimir)).
* Backported in [#26143](https://github.com/ClickHouse/ClickHouse/issues/26143): Fix possible crash in `pointInPolygon` if the setting `validate_polygons` is turned off. [#26113](https://github.com/ClickHouse/ClickHouse/pull/26113) ([Alexey Milovidov](https://github.com/alexey-milovidov)).