Adjust SDK Diff Pipeline Resource Trigger to Improve Frequency

[ellahathaway]

Related to #5151:

The SDK Diff pipeline is currently configured to trigger only on successful release branch builds of the dotnet-unified-build pipeline. However, due to the low success rate of dotnet-unified-build pipeline runs, the SDK Diff pipeline is not running frequently. For example, in this build, all build stages succeeded, but the SDK Diff pipeline did not trigger because the final VMR validation stage failed.

To address this, we should consider adjusting the resource trigger so that the SDK Diff pipeline fires after the SB build and VMR final join stages complete successfully.

[NikolaMilosavljevic]

Is the expectation to run this pipeline for every unified-build that has successful VMR Final Join and source-build stages? I've checked the last two weeks of pipeline runs in main - there was at least one run for every work day.

[ellahathaway]

I see us continuing to use the schedule for runs on main.

My thinking is that this change would only apply to the current pipeline resource triggers for release & internal/release branches.