chore: add lean4-cli to release checklist (#6596)

Users have requested toolchain tags on `lean4-cli`, so let's add it to
the release checklist to make sure these get added regularly.

Previously, `lean4-cli` has used more complicated tags, but going
forward we're going to just use the simple `v4.16.0` style tags, with no
repository-specific versioning.

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Co-authored-by: Markus Himmel <markus@lean-fro.org>