refactor endpointSast in app/views/projects/merge_requests/_page.html.haml to boolean

This Issue is a follow up to: !134158 (merged) After refactoring the SAST Request from REST to GraphQL a defined passing an endpoint to the client in https://gitlab.com/gitlab-org/gitlab/-/blob/master/app/views/projects/merge_requests/_page.html.haml#L101 is no longer needed.

Instead the client only needs to know whether or not a SAST report is available for the MR.

Therefore endpointSast which is a String should be refactored to sastReportAvailable as a boolean.

Once this resolved we need to remove the frontend workaround: https://gitlab.com/gitlab-org/gitlab/-/blob/5aa1bba06f9e3a7ccb414aa6e50772722d9db3dd/app/assets/javascripts/diffs/index.js#L40