Replace Fabric Bot automation for requiring reviews of PRs that cange the .NET SDK version

This repository has had automation in place that prevents auto-merge of PRs that change the .NET SDK version. That automation was implemented through the Fabric Bot tooling with the following logic:

1. When a pull request is opened
2. If the body of the pull requst contains the string, "Updates sdk.version"
3. And the author of the PR is "dotnet-maestro[bot]"
4. Then issue the action of requiring changes to the pull request, leaving a comment of, "This PR changes the .NET SDK version. Review required before merging."

We need to remove this automation as our migration from Fabric Bot to the Policy Service Resource Management tooling will not support the automated action of requesting changes to a pull request.

After the migration to the new service is completed, we can reintroduce this automation, perhaps with the approach of using the same logic as above, but issuing the action of disabling auto-merge, either through removing the label or using GitHub's built-in auto-merge and disabling that.

/cc @arkalyanms @wtgodbe @mkArtakMSFT @jaredpar