Fix or remove the whitesource check from non-main branches where it is unreliable

[joshuarrrr]

On non-main branches, the whitesource check is marked as 'failed' even when no scan was triggered. For example, see https://github.com/opensearch-project/OpenSearch-Dashboards/pull/3699/checks?check_run_id=12310441779

In main, such cases are not marked as failures: https://github.com/opensearch-project/OpenSearch-Dashboards/pull/3652/checks?check_run_id=12258568680

We need to fix this behavior, or limit the check to only main.

[AMoo-Miki]

I have a feeling that Mend is failing intentionally so we will not call them with every change unnecessarily; We should the Mend worflows to only when the lockfile or the package manifests change; we need the package manifests for cases where a version was bumped but a lock file was accidentally (or maliciously) omitted.

In short: we certainly want Mend scans run on all non-main branches; we just need to be frugal and so only when needed.