cli: deprecate `jj checkout` and `jj merge` #8794

Job	Run time
build (ubuntu-latest)	5m 16s
build (macos-14)	3m 14s
Check protos	36s
build (windows-latest)	7m 35s
Check that MkDocs can build the docs with Poetry 1.3.2	43s
build (ubuntu-latest, --all-features)	6m 2s
Check that MkDocs can build the docs	25s
Check formatting	15s
cargo-deny (advisories)	25s
cargo-deny (bans licenses sources)	28s
Clippy check	2m 19s
	27m 18s

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