MenuToggle - use new text modifier for split action toggle button

Follow-up to https://github.com/patternfly/patternfly-react/pull/10256.

Remove custom styling from MenuToggle for the split action toggle button text and use the new modifier `pf-m-text` added in https://github.com/patternfly/patternfly/pull/6668 (v5) / https://github.com/patternfly/patternfly/pull/6886 (v6).