Rename taxonomy kinds from taxonomy to term, taxonomyTerm to taxonomy

And we have taken great measures to limit potential site breakage:

* For `disableKinds` and `outputs` we try to map from old to new values if possible, if not we print an ERROR that can be toggled off if not relevant.
* The layout lookup is mostly compatible with more options for the new `term` kind.

That leaves:

* Where queries in site.Pages using taxonomy/taxonomyTerm Kind values as filter.
* Other places where these kind value are used in the templates (classes etc.)

Fixes #6911
Fixes #7395

hugolib/testhelpers_test.go

@@ -1070,3 +1070,18 @@ func captureStderr(f func() error) (string, error) {
 	io.Copy(&buf, r)
 	return buf.String(), err
 }
+
+func captureStdout(f func() error) (string, error) {
+	old := os.Stdout
+	r, w, _ := os.Pipe()
+	os.Stdout = w
+
+	err := f()
+
+	w.Close()
+	os.Stdout = old
+
+	var buf bytes.Buffer
+	io.Copy(&buf, r)
+	return buf.String(), err
+}