More automation for updating dependencies

- Convert update checks in check.sh to mechanical updates.
  - Use https://www.updatecli.io/ for now, which is not as
    full-featured as renovatebot or dependabot, but I found it easier
    to plug arbitrary shell scripts into.
- Add updaters for
  - ubuntu-latest-lts (which is similar to GitHub Action's "ubuntu-latest").
  - FreeBSD image used in Cirrus (requires "gcloud auth login" for now,
    see https://github.com/cirruslabs/cirrus-ci-docs/issues/1315)
  - littlecheck and widecharwidth
- Update all dependencies except Cargo ones.
- As a reminder, our version policies are arbitrary and can be changed
  as needed.
- To-do:
  - Add updaters for GitHub Actions (such as "actions/checkout").
    Renovatebot could do that.

.github/actions/rust-toolchain/action.yml

@@ -25,8 +25,8 @@ runs:
         set -x
         toolchain=$(
           case "$toolchain_channel" in
-            (stable) echo 1.91 ;;
-            (msrv)   echo 1.85 ;;
+            (stable) echo 1.91 ;; # updatecli.d/rust.yml
+            (msrv)   echo 1.85 ;; # updatecli.d/rust.yml
             (*)
               printf >&2 "error: unsupported toolchain channel %s" "$toolchain_channel"
               exit 1