ogcapi-images: Avoid ServiceException in WFS GetCapabilities

`STACItemFeaturesResponse.getCapabilitiesElementName()` throws
an `UnsupportedOperationException` instead of returning `null`,
producing a `ServiceExceptionReport` when requesting a WFS 1.0.0
`GetCapabilities` document.

`null` is a valid return value for this method, that results
in `WFSGetFeatureOutputFormat.getCapabilitiesElementNames()`
not contributing an output format name for the
`WFSGetFeatureOutputFormat` in question.

src/community/ogcapi/ogcapi-images/src/main/java/org/geoserver/api/images/STACItemFeaturesResponse.java

@@ -273,9 +273,15 @@ protected void writeCollectionBounds(
         // not needed in WFS3
     }
 
-    /** capabilities output format string. */
+    /**
+     * {@inheritDoc}
+     *
+     * @return {@code null}, making {@link WFSGetFeatureOutputFormat#getCapabilitiesElementNames()}
+     *     not contributing a result format on the GetCapabilities document for this output format.
+     */
+    @Override
     public String getCapabilitiesElementName() {
-        throw new UnsupportedOperationException();
+        return null;
     }
 
     @Override