Prepare 2.4.2rc0 (pantsbuild#12127)

[ci skip-rust]

src/python/pants/notes/2.4.x.md

@@ -2,6 +2,16 @@
 
 See https://www.pantsbuild.org/v2.4/docs/release-notes-2-4 for an overview of the changes in this release series.
 
+## 2.4.2rc0 (May 25, 2021)
+
+### Performance
+
+* Halve memory usage for remote cache writes (Cherry-pick of #12083) ([#12118](https://github.com/pantsbuild/pants/pull/12118))
+
+### Documentation
+
+* Rewrite error message when Pantsd is shut down during run (Cherry-pick of #12107) ([#12117](https://github.com/pantsbuild/pants/pull/12117))
+
 ## 2.4.1 (May 6, 2021)
 
 The second stable release of the `2.4.x` series, with no changes since the previous `rc`.