Remove pre-release Julia from CI_extensions

ModelingToolkitSIExt tests hang on Julia pre-release (1.12-DEV),
causing a 6-hour timeout. This is an upstream issue with
ModelingToolkit and its dependencies on pre-release Julia. The main
CI workflow still tests Core and QA groups on pre-release.

Co-Authored-By: Chris Rackauckas <accounts@chrisrackauckas.com>
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: claude

.github/workflows/CI_extensions.yml

@@ -15,7 +15,6 @@ jobs:
           - ModelingToolkitSIExt
         version:
           - '1'
-          - 'pre'
     steps:
       - uses: actions/checkout@v6
       - uses: julia-actions/setup-julia@v2