Add dependabot.yml for dependency version updates #4057

Job	Run time
cargo-deny (advisories)	41s
cackle	1m 6s
check-git-rev-deps	6s
build-and-test (msrv, 20, ubuntu-latest, wasm32-unknown-unknown, false)	6m 6s
cargo-deny (bans licenses sources)	40s
build-and-test (msrv, 20, ubuntu-latest, x86_64-unknown-linux-gnu, true)	16m 11s
fmt	6s
publish-dry-run	0s
build-and-test (msrv, 21, ubuntu-latest, wasm32-unknown-unknown, false)	5m 58s
semver-checks	3m 56s
build-and-test (msrv, 21, ubuntu-latest, x86_64-unknown-linux-gnu, true)	16m 2s
build-and-test (latest, 20, ubuntu-latest, wasm32-unknown-unknown, false)	5m 35s
build-and-test (latest, 20, ubuntu-latest, x86_64-unknown-linux-gnu, true)	14m 54s
build-and-test (latest, 21, ubuntu-latest, wasm32-unknown-unknown, false)	5m 46s
build-and-test (latest, 21, ubuntu-latest, x86_64-unknown-linux-gnu, true)	14m 44s
complete	1s
	1h 31m 52s

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