fix(richtext-lexical): use headingLevel in danish heading label (#15685)

### What?

Danish rich text heading labels now show "Overskrift 1", "Overskrift 2",
etc. instead of the raw placeholder "Overskrift {{overskriftNi...".

### Why?

With the UI in Danish, the heading dropdown (H1–H4) showed an unresolved
placeholder because the Danish string used `{{overskriftNiveau}}` while
the code only passes `headingLevel`. The interpolation key must be
`{{headingLevel}}` for the value to be substituted.

### How?

In `packages/richtext-lexical/src/features/heading/server/i18n.ts`, the
Danish (`da`) label was changed from `'Overskrift {{overskriftNiveau}}'`
to `'Overskrift {{headingLevel}}'` so it matches the variable passed by
the client. No other locales used a different placeholder.

Author: divyesh123-jain

packages/richtext-lexical/src/features/heading/server/i18n.ts

@@ -14,7 +14,7 @@ export const i18n: Partial<GenericLanguages> = {
     label: 'Nadpis {{headingLevel}}',
   },
   da: {
-    label: 'Overskrift {{overskriftNiveau}}',
+    label: 'Overskrift {{headingLevel}}',
   },
   de: {
     label: 'Überschrift {{headingLevel}}',