OZG-6237 Replace component with heading styles

alfa-client/apps/admin/src/styles.scss

...t-container/organisationseinheit-container.component.html

...fach-container/postfach-form/postfach-form.component.html

alfa-client/libs/design-system/.storybook/styles.scss

alfa-client/libs/design-system/src/index.ts

...s/design-system/src/lib/heading/heading.component.spec.ts

...t/libs/design-system/src/lib/heading/heading.component.ts

...ent/libs/design-system/src/lib/heading/heading.stories.ts