www/rt60: Update to 6.0.3

This release fixes the following vulnerabilities:
CVE-2026-44231
CVE-2026-41075
CVE-2026-41076
CVE-2026-41074
CVE-2026-44228
CVE-2026-6841
CVE-2026-44227
CVE-2026-44230
CVE-2026-44229
CVE-2026-41073

Changelog: https://github.com/bestpractical/rt/releases/tag/rt-6.0.3

PR: 295470
Approved by: Einar Bjarni Halldórsson (maintainer)