ci(bors): set update_base_for_deletes (#17410)

From the source, this doesn't do anything to the git history, but does prevent chained PRs being closed after bors closes the first one.

This still doesn't work very well for us, but it's better than randomly closing PRs that haven't been merged.

bors.toml

@@ -6,4 +6,5 @@ use_squash_merge = true
 timeout_sec = 28800
 block_labels = ["not-ready-to-merge", "WIP", "blocked-by-other-PR", "merge-conflict", "awaiting-CI"]
 delete_merged_branches = true
+update_base_for_deletes = true
 cut_body_after = "---"