Avoid `func` prefix in function name

As decided in
https://github.com/PHPOffice/PhpSpreadsheet/pull/1954#issuecomment-809824463

src/PhpSpreadsheet/Calculation/Calculation.php

@@ -2314,7 +2314,7 @@ class Calculation
         ],
         'SUM' => [
             'category' => Category::CATEGORY_MATH_AND_TRIG,
-            'functionCall' => [MathTrig\Sum::class, 'funcSumNoStrings'],
+            'functionCall' => [MathTrig\Sum::class, 'sumErroringStrings'],
             'argumentCount' => '1+',
         ],
         'SUMIF' => [

src/PhpSpreadsheet/Calculation/Database/DSum.php

@@ -38,7 +38,7 @@ class DSum extends DatabaseAbstract
             return null;
         }
 
-        return MathTrig\Sum::funcSum(
+        return MathTrig\Sum::sumIgnoringStrings(
             self::getFilteredColumn($database, $field, $criteria)
         );
     }

src/PhpSpreadsheet/Calculation/MathTrig.php

@@ -771,8 +771,8 @@ class MathTrig
      *
      * @Deprecated 1.18.0
      *
-     * @See MathTrig\Sum::funcSumNoStrings()
+     * @See MathTrig\Sum::sumErroringStrings()
-     *      Use the funcSumNoStrings method in the MathTrig\Sum class instead
+     *      Use the sumErroringStrings method in the MathTrig\Sum class instead
      *
      * Excel Function:
      *        SUM(value1[,value2[, ...]])
@@ -783,7 +783,7 @@ class MathTrig
      */
     public static function SUM(...$args)
     {
-        return MathTrig\Sum::funcSum(...$args);
+        return MathTrig\Sum::sumIgnoringStrings(...$args);
     }
 
     /**

src/PhpSpreadsheet/Calculation/MathTrig/Subtotal.php

@@ -60,7 +60,7 @@ class Subtotal
         [Product::class, 'evaluate'], // 6
         [Statistical\StandardDeviations::class, 'STDEV'], // 7
         [Statistical\StandardDeviations::class, 'STDEVP'], // 8
-        [Sum::class, 'funcSum'], // 9
+        [Sum::class, 'sumIgnoringStrings'], // 9
         [Statistical\Variances::class, 'VAR'], // 10
         [Statistical\Variances::class, 'VARP'], // 11
     ];

src/PhpSpreadsheet/Calculation/MathTrig/Sum.php

@@ -18,7 +18,7 @@ class Sum
      *
      * @return float|string
      */
-    public static function funcSum(...$args)
+    public static function sumIgnoringStrings(...$args)
     {
         $returnValue = 0;
 
@@ -47,7 +47,7 @@ class Sum
      *
      * @return float|string
      */
-    public static function funcSumNoStrings(...$args)
+    public static function sumErroringStrings(...$args)
     {
         $returnValue = 0;
         // Loop through the arguments