Rebootstrap pipeline should accept a target branch parameter

A while back, I had to rebootstrap the SDK in the `main` branch using artifacts from the `preview7` branch. When I ran the rebootstrap pipeline, the rebootstrap PR was opened targeting the `preview7` branch. This wasn't an issue for this particular rebootstrap since `preview7` also needed to be rebootstrapped. However, it highlighted a problem: if we need to rebootstrap a branch using artifacts from a build of another branch, we can't do that via the rebootstrap pipeline. To resolve this, the rebootstrap pipeline should take a parameter that accepts a target branch. The default of this parameter should be the branch that the artifacts were pulled from.