docs: prepare changelog for v1.21.0 release

CHANGELOG.md

@@ -1,5 +1,6 @@
 # Table of Contents
 
+- [v1.21.0](#v1210)
 - [v1.20.0](#v1200)
 - [v1.19.1](#v1191)
 - [v1.19.0](#v1190)
@@ -58,14 +59,26 @@
 
 ## [v1.21.0]
 
-> Release date: TBD
+> Release date: 2023/05/31
 
 ### Add
 
 - Add support for updating Services, Routes, and Consumers by changing their IDs,
   but retaining their names.
   [#918](https://github.com/Kong/deck/pull/918)
 
+### Fix
+
+- Return proper error when HTTP calls fail on validate.
+  [#869](https://github.com/Kong/deck/pull/869)
+- Replace old docs link in `convert` and fix its docstring.
+  [#905](https://github.com/Kong/deck/pull/905)
+
+### Misc
+
+- Bump Go toolchain to 1.20.
+  [#898](https://github.com/Kong/deck/pull/898)
+
 ## [v1.20.0]
 
 > Release date: 2023/04/24
@@ -1211,6 +1224,7 @@ No breaking changes have been introduced in this release.
 
 Debut release of decK
 
+[v1.21.0]: https://github.com/kong/deck/compare/v1.20.0...v1.21.0
 [v1.20.0]: https://github.com/kong/deck/compare/v1.19.1...v1.20.0
 [v1.19.1]: https://github.com/kong/deck/compare/v1.19.0...v1.19.1
 [v1.19.0]: https://github.com/kong/deck/compare/v1.18.1...v1.19.0