[usability] Update backport action to use final commit

[dougbu]

The backport action opens a PR with all of the commits in the original, even if the original was squashed and merged. Means squashing of the final PR is effectively required. We should prioritize finding the final commit and cherry-pick that SHA if it's available.

See for example #33033 which opened with multiple commits from #32812 and required additional commits to build successfully in release/5.0. Without an interactive git rebase, those commits will remain separate if we rebase or merge and be inseparable from other work if we squash.

[dougbu]

/fyi @akoeplinger @dotnet/aspnet-build

[dougbu]

@wtgodbe, @mkArtakMSFT suggest closing this one

[dougbu]

Fixed typo above