Backport of chore: Update changelog for 0.20.3 release into release/0.20.x (#6654)

* backport of commit f4d1261116

* backport of commit 7531d3c96d

* backport of commit 797d58d5c7

---------

Co-authored-by: Ryan Derr <ryan.derr@hashicorp.com>
Co-authored-by: Ryan Derr <54389874+RyanDerr@users.noreply.github.com>

CHANGELOG.md

@@ -6,6 +6,9 @@ Canonical reference for changes, improvements, and bugfixes for Boundary.
 
 ## 0.20.3 (2026/04/24)
 
+### New and Improved 
+* Added support for new `debug` flag to expose pprof endpoints for debugging purposes. ([PR](https://github.com/hashicorp/boundary/pull/6644))
+
 ### Security
 
 * Updated jackc/pgx/v5 dependency to v5.9.2 to address GHSA-j88v-2chj-qfwx, GO-2026-4771, GO-2026-4772, and GHSA-9jj7-4m8r-rfcm ([PR](https://github.com/hashicorp/boundary/pull/6607), [PR](https://github.com/hashicorp/boundary/pull/6617))