update changelog

Author: Doug Bell

docs/changelog/7.x.x.txt

@@ -1,5 +1,7 @@
 7.10.20
  - fixed: Do not call group methods on an undefined value.
+ - fixed #12178: random deletion of columns may happen when a schema is saved
+ - fixed #12179: DataTable Field types get reset to "Text" in Edit Schema
 
 7.10.19
  - fixed #12169: extras uploads symlink export