Replace gettype() with get_debug_type() in exception messages etc.

get_debug_type() does the same thing but better (spelling type names
in the same way as in type declarations, and including names of
object classes and resource types). It was added in PHP 8, but the
symfony/polyfill-php80 package provides it while we still support 7.4.

For reference:
https://www.php.net/manual/en/function.get-debug-type.php
https://www.php.net/manual/en/function.gettype.php

Change-Id: I0ad6d2535c0531badf02ee8643bd7294deea0b5f

Author: MatmaRex

src/SiteLinkList.php

@@ -143,7 +143,7 @@ public function getBySiteId( $siteId ) {
 	 */
 	public function hasLinkWithSiteId( $siteId ) {
 		if ( !is_string( $siteId ) ) {
-			throw new InvalidArgumentException( '$siteId must be a string; got ' . gettype( $siteId ) );
+			throw new InvalidArgumentException( '$siteId must be a string; got ' . get_debug_type( $siteId ) );
 		}
 
 		return array_key_exists( $siteId, $this->siteLinks );
@@ -193,7 +193,7 @@ public function toArray() {
 	 */
 	public function removeLinkWithSiteId( $siteId ) {
 		if ( !is_string( $siteId ) ) {
-			throw new InvalidArgumentException( '$siteId must be a string; got ' . gettype( $siteId ) );
+			throw new InvalidArgumentException( '$siteId must be a string; got ' . get_debug_type( $siteId ) );
 		}
 
 		unset( $this->siteLinks[$siteId] );