CCM-10548: updated aria tag

Author: mark-r-bjss

frontend/src/__tests__/components/forms/LetterTemplateForm.test.tsx/__snapshots__/LetterTemplateForm.test.tsx.snap

@@ -2247,7 +2247,7 @@ exports[`renders page with preloaded field values 1`] = `
             </select>
           </div>
           <div
-            aria-live="assertive"
+            aria-live="polite"
             class="nhsuk-warning-callout"
             data-testid="rtl-language-warning"
           >
@@ -2700,7 +2700,7 @@ exports[`shows right-to-left language warning when language changes 1`] = `
             </select>
           </div>
           <div
-            aria-live="assertive"
+            aria-live="polite"
             class="nhsuk-warning-callout"
             data-testid="rtl-language-warning"
           >

frontend/src/components/forms/LetterTemplateForm/LetterTemplateForm.tsx

@@ -183,7 +183,7 @@ export const LetterTemplateForm: FC<
                 isRightToLeft(letterTemplateLanguage) && (
                   <WarningCallout
                     data-testid='rtl-language-warning'
-                    aria-live='assertive'
+                    aria-live='polite'
                   >
                     <WarningCallout.Label>
                       {rtlWarning.heading}