base: Fix release date in changelog

libraries/base/changelog.md

 # Changelog for [`base` package](http://hackage.haskell.org/package/base)
 
-## 4.20.0.0 *TBA*
+## 4.20.0.0 *May 2024*
+  * Shipped with GHC 9.10.1
   * Deprecate `GHC.Pack` ([#21461](https://gitlab.haskell.org/ghc/ghc/-/issues/21461))
   * Export `foldl'` from `Prelude` ([CLC proposal #167](https://github.com/haskell/core-libraries-committee/issues/167))
   * The top-level handler for uncaught exceptions now displays the output of `displayException` rather than `show`  ([CLC proposal #198](https://github.com/haskell/core-libraries-committee/issues/198))