Add UXP v2.1.4-up.3 release notes (#1130)

* Add UXP v2.1.4-up.3 release notes

Adds release notes for UXP v2.1.4-up.3, which includes FunctionRunner
payload size metrics backported to the 2.1 release branch.

Signed-off-by: Philippe Scorsolini <5697904+phisco@users.noreply.github.com>

* Remove links from release notes

Signed-off-by: Philippe Scorsolini <5697904+phisco@users.noreply.github.com>

---------

Signed-off-by: Philippe Scorsolini <5697904+phisco@users.noreply.github.com>

Author: phisco

docs/reference/release-notes/uxp.md

@@ -14,6 +14,14 @@ Any important warnings or necessary information
 - User-facing changes
 -->
 
+## v2.1.4-up.3
+
+### Release Date: 2026-04-08
+
+#### What's Changed
+- Bumped Crossplane to v2.1.4-up.3
+- Added FunctionRunner payload size metrics
+
 ## v2.2.0-up.3
 
 ### Breaking changes