feat: add dedicated `prune_master_keys` to remove old subkeys #1671

Job	Run time
cargo-lint / cargo-dry-publish / publish	24s
cargo-lint / cargo-doc / doc	28s
cargo-lint / cargo-semver / Check semver	34s
cargo-lint / cargo-nursery-base / cargo-audit / Audit	5m 10s
cargo-lint / cargo-nursery-base / cargo-lint / lint	1m 34s
cargo-lint / cargo-nursery-base / cargo-udeps / Check udeps	56s
cleanup	0s
cargo-publish	0s
	9m 6s

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