STF-322: Bump CHANGELOG version to 5.1.0 per review

Reviewer pointed out that adding a public Builder API and changing the
default retry behavior is a feature-grade change, not a patch. Bump the
in-progress version header from 5.0.3 to 5.1.0 to match semver intent.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: oschwald

CHANGELOG.md

@@ -1,7 +1,7 @@
 CHANGELOG
 =========
 
-5.0.3 (unreleased)
+5.1.0 (unreleased)
 ------------------
 
 * Added `WebServiceClient.Builder.maxRetries(int)` to bound transport-failure