Sync from upstream on a timed schedule

[tedjpoole]

This PR makes the envoy-sync-receive.yaml workflow a no-op, because we don't want to be triggered too often by the high rate of upstream commits. However, it is left in place so that
(1) it saves having to modify the upstream envoy-sync.yaml workflow, to remove the dispatch code, and
(2) we can more easily reinstate the push behaviour if/when needed (e.g. on just release/v1.xx branches, but not main).

The new envoy-sync-scheduled.yaml workflow performs a sync from upstream on a timed schedule instead, allowing us to better control when and how often synchronisation is performed thus reducing the noise and the number of (expensive) builds.

[tedjpoole]

Note that this targets the release/v1.28 branch because that is the current default branch, and scheduled workflows only run from the default branch.

[tedjpoole]

Also note that for some (not really debugged yet) reason, upstream was only ever invoking our envoy-sync-receive.yaml workflow for the main branch, so the release/v1.xx branches were effectively not automated.
However, the new envoy-sync-scheduled.yaml workflow does automate the synchronisation of all specified branches as expected.