Prepare release 6.0.3

[mparticle-automation]

Preparing for release 6.0.3 from main.

• Bumped version to 6.0.3

[cursor]

PR Summary

Low Risk
Version and changelog-only release prep with no runtime code changes in this diff.

Overview
Release 6.0.3 (2026-08-28): bumps the canonical version from 6.0.2 to 6.0.3 in VERSION and adds a dated 6.0.3 section to CHANGELOG.md.

The changelog records what ships in this release (not changed in this PR): Core adds support for native GoDaddy R1 certificate chains ([#766]); Rokt kit reports mParticle API usage ([#761]).

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