MDL-71169 external: Use execute as the default external method name

If the 'methodname' is not explicitly specified in db/services.php,
expect it to be 'execute'.

lib/upgrade.txt

@@ -118,6 +118,8 @@ information provided here is intended especially for developers.
 * The behat transformation 'string time to timestamp' no longer supports datetime format. If provided, the format must
   be strftime compatible. Example:
     - I should see "##tomorrow noon##%A, %d %B %Y, %I:%M %p##"
+* External functions implementation classes should use 'execute' as the method name, in which case the
+  'methodname' property should not be specified in db/services.php file.
 
 === 3.10 ===
 * PHPUnit has been upgraded to 8.5. That comes with a few changes:

lib/upgradelib.php

@@ -1211,6 +1211,7 @@ function external_update_descriptions($component) {
         $function = $functions[$dbfunction->name];
         unset($functions[$dbfunction->name]);
         $function['classpath'] = empty($function['classpath']) ? null : $function['classpath'];
+        $function['methodname'] = $function['methodname'] ?? 'execute';
 
         $update = false;
         if ($dbfunction->classname != $function['classname']) {
@@ -1260,7 +1261,7 @@ function external_update_descriptions($component) {
         $dbfunction = new stdClass();
         $dbfunction->name       = $fname;
         $dbfunction->classname  = $function['classname'];
-        $dbfunction->methodname = $function['methodname'];
+        $dbfunction->methodname = $function['methodname'] ?? 'execute';
         $dbfunction->classpath  = empty($function['classpath']) ? null : $function['classpath'];
         $dbfunction->component  = $component;
         $dbfunction->capabilities = array_key_exists('capabilities', $function)?$function['capabilities']:'';