Bump @actions/core from 1.11.1 to 2.0.3

v3.0.0 is ESM-only which is incompatible with the current ncc/CJS build
setup. Use v2.0.3 instead which adds Node 24 support while remaining
CJS-compatible.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: hsbt