CI: a nightly full run on main and release/10.x #343

Problem

Some CI inputs change without any diff:

• the postgres:16-alpine and postgres:18-alpine tags
• the ubuntu-latest image
• the nuget.org packages the smoke consumers restore (EFCore.ComplexIndexes.PostgreSQL on 10.x)
• the SDK band that global.json's rollForward: latestFeature admits

CI runs only when something is pushed. release/10.x can go weeks without a push, so the first sign of a drift is a release day, or a PR that happens to land at the time.

Change

• Add a schedule trigger to dotnet.yml. A scheduled workflow runs only on the default branch, so it runs every lane on main.
• Add a step that dispatches dotnet.yml on release/10.x (gh workflow run dotnet.yml --ref release/10.x; needs actions: write).
• GitHub turns off schedules in a public repository after 60 days without activity, so nothing release-critical may depend on the nightly. Release: run every CI lane (PostgreSQL 16 and 18, both smoke modes, the EF float legs) before publishing #342 makes the release run everything itself.

Done when

Nightly runs appear for main and release/10.x, every lane runs in each, and a failed nightly notifies the maintainer.