CI: Enable sync-labels for labeler

The `labeler` job has an input `sync-labels` (default `false`) that
configures:

> Whether or not to remove labels when matching files are reverted or no
> longer changed by the PR

ref: https://github.com/actions/labeler

Currently we are using the default which means labels are not synced
when a PR is modified.

Example

A PR initially touches the `hashes` and `bitcoin` crates and the labeler
action adds `C-hashes` and `C-bitcoin`. Later, a force push removes the
changes to `hashes`, the labeler will not remove the `C-hashes` label.
Now the labels are incorrect.

Note on `pull_request_target`:

We use `on: pull_request_target`, this means the action is run in the
target branch not in the PR branch. So the changes in this patch are not
run when this PR is pushed up. Therefore we cannot test this patch but
have to merge it blindly - please review carefully.

Some further security notes that I learned while doing this patch. The
`pull_request_target` has access to GitHub secrets, and in
`manage-pr.yaml` we run `./contrib/gen_label_config.sh`. This is safe
because its the version of this script on `master` that is run not the
version in the PR (so a malicious attacker cannot patch
`gen_label_config.sh` and have it be run). However we need to be aware
that we do not accidentally merge some funky shell and inadvertently
leak secrets.

.github/workflows/manage-pr.yml

@@ -24,3 +24,4 @@ jobs:
         uses: actions/labeler@v5
         with:
           configuration-path: master/.github/labeler.yml
+          sync-labels: true