Align class and file names

Rename `Exercise` class to `LargestSeriesProduct` (matches file name and the class name expected by the tests

exercises/practice/largest-series-product/src/main/groovy/LargestSeriesProduct.groovy

@@ -1,5 +1,5 @@
-class Exercise {
+class LargestSeriesProduct {
     static largestProduct(digits, span) {
         throw new UnsupportedOperationException('Method implementation is missing')
     }
-}
+}