Fix downgrade workflow trigger for main

[ChrisRackauckas-Claude]

Please ignore this PR until it has been reviewed by @ChrisRackauckas.

Summary

Point the downgrade workflow's push trigger at the repository's actual default branch, main, instead of the retired master name.

The pull-request trigger and concurrency rule already used main; only pushes to the default branch were silently missing downgrade coverage.

Local verification

• Reproduced the deployed downgrade workflow locally at commit a89cd5e5 using Julia 1.10.11 and julia-downgrade-compat fab1def: 37/37 assertions passed.
• Pkg.build() passed in the exact floor environment.
• actionlint .github/workflows/Downgrade.yml passed.
• Runic 1.7 passed for all 16 Julia files.
• git diff --check passed.