ci: use the shared scheduled-failure report

[lesnik512]

Switches the scheduled-failure report to the shared workflow, per the updated standard CI2 (modern-python/.github#127).

Changes

• scheduled.yml's report-failure job now calls modern-python/.github/.github/workflows/report-scheduled-failure.yml@main, the job CI2 requires byte for byte.
• .github/scripts/report-scheduled-failure.sh is deleted. CI2 forbids a per-repo copy.

The shared workflow keeps the label, issue title and open-or-comment behaviour. Only the issue text changes: it no longer lists likely culprit packages, since the per-repo lists had drifted. It points at the linked run instead.

Verification

• actionlint 1.7.12 passes on scheduled.yml.
• The shared workflow was tested end to end from its branch in eof-fixer before #127 merged: the first run opened an issue, and the second commented on it. See feat: share the scheduled-failure report as a reusable workflow .github#127.
• The new job only runs on a scheduled failure, so this PR's CI can't exercise it.