OZG-8129 replace pdf export icon

alfa-client/apps/alfa/src/app/app.component.spec.ts

alfa-client/apps/alfa/src/app/app.component.ts

alfa-client/libs/design-system/src/assets/pdf-export.svg

...h-mail-pdf-button/postfach-mail-pdf-button.component.html

...ail-pdf-button/postfach-mail-pdf-button.component.spec.ts

alfa-client/libs/ui/src/index.ts

alfa-client/libs/ui/src/lib/assets/pdf_export.svg

alfa-client/libs/ui/src/lib/icon/icon.model.ts

alfa-client/libs/ui/src/lib/icon/icon.service.spec.ts

alfa-client/libs/ui/src/lib/icon/icon.service.ts