artifact: honor Retry-After on HTTP 429 and prepare 6.3.0

Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>

RELEASES.md

@@ -1,5 +1,10 @@
 # @actions/artifact Releases
 
+## 6.3.0
+
+- Honor positive integer `Retry-After` seconds on HTTP 429 responses, including when the response body is not valid JSON. Unsupported or missing values use exponential backoff.
+- Increase the initial fallback retry wait to 8 seconds and limit accumulated retry sleep to 120 seconds.
+
 ## 6.2.2
 
 - Bump dependency versions ([#2393](https://github.com/actions/toolkit/pull/2393)):