Rename `DatasetName::dataset()` to `DatasetName::kind()`

[jgallagher]

We've noted this in a TODO:

omicron/common/src/disk.rs

Lines 110 to 113 in ac3fb34

	// TODO: Maybe rename this to "kind"?
	pub fn dataset(&self) -> &DatasetKind {
	&self.kind
	}

and @andrewjstone noted confusion in #7104 (comment). It would be a little disruptive to make that change as a part of that PR; opening this to remind me to go back and actually do this.