ci: dispatch beta release via execute-workflow instead of inlining (#890)

Replaces the inlined beta release jobs in `on_master.yaml` with a single
job that dispatches `manual_release_beta.yaml` via
`apify/workflows/execute-workflow@main`. The dispatched run is a regular
`workflow_dispatch` (not a reusable workflow call), so PyPI's Trusted
Publishing accepts the OIDC token, and the duplication is gone.

Also adds a `concurrency: release` group to `manual_release_beta.yaml`
so two rapid pushes to master cannot race on the version bump and PyPI
publish.

Same pattern as `apify-client-js`, `apify-sdk-js`, `apify-cli`, and
`crawlee`.