[Breaking Change] the cross-version check, when applicable, should compare not only against `main` but also against `RPSaaSMaster`

Context from [this Teams discussion](https://teams.microsoft.com/l/message/19:78cd6c72c41240dc84a36523af60b88a@thread.tacv2/1701990974477?tenantId=72f988bf-86f1-41af-91ab-2d7cd011db47&groupId=0cab4ce9-7691-42ae-82e3-460d4346a710&parentMessageId=1701892274551&teamName=ARM%20API%20Reviewers&channelName=ARM%20swagger%20reviews&createdTime=1701990974477):

There is this PR: [Tshaiman vi microsoft.video indexer 2023 11 01 preview by tshaiman · Pull Request #16176 · Azure/azure-rest-api-specs-pr (github.com)](https://github.com/Azure/azure-rest-api-specs-pr/pull/16176), where the automation is taking some API-versions from "main" branch for checking breaking changes,
whereas the author is aiming to merge it into RPSaaSMaster branch in private repo.

Shouldn't it use "RPSaaSMaster" branch's latest preview and stable version for comparison, instead of "main" branch?

Example:

Current main branch for this author:
Latest Preview API Version in the main branch: [2022-07-20-preview](https://github.com/Azure/azure-rest-api-specs-pr/tree/main/specification/vi/resource-manager/Microsoft.VideoIndexer/preview/2022-07-20-preview)
Latest Stable API Version in the main branch: [2022-08-01](https://github.com/Azure/azure-rest-api-specs-pr/tree/main/specification/vi/resource-manager/Microsoft.VideoIndexer/stable/2022-08-01)

Current RPSaaSMaster branch looks like:
Latest Preview API Version in the RPSaaSMaster branch: [2023-09-01-preview](https://github.com/Azure/azure-rest-api-specs-pr/tree/RPSaaSMaster/specification/vi/resource-manager/Microsoft.VideoIndexer/preview/2023-09-01-preview)
Latest Stable API Version in the RPSaaSMaster branch: N/A

The author created the PR with this commit: [Adds base for updating Microsoft.VideoIndexer from version preview/20…](https://github.com/Azure/azure-rest-api-specs-pr/pull/16176/commits/2dbdc8b4e608cc3050fc1b7afe71d9508ce02b00) …
which used base branch as "2023-09-01-preview".

@mikekistler said:
> I agree that there should be checks against the earlier preview in RPSaaSMaster -- in addition to the checks against main.

Example cross-version breaking change check run:
https://github.com/Azure/azure-rest-api-specs-pr/pull/16176/checks?check_run_id=19425829486

ADO log:
https://dev.azure.com/azure-sdk/internal/_build/results?buildId=3326326&view=logs&j=8f628bfe-7d8d-5bf3-e141-37106862c4f5&t=723dfc11-d28f-5d74-45f1-663cde6746b4