used native PHP 8 functions

Author: dg

src/Mail/DkimSigner.php

@@ -104,7 +104,7 @@ protected function computeSignature(string $rawHeader, string $signature): strin
 
 		$parts = [];
 		foreach ($test = explode("\r\n", $rawHeader) as $key => $header) {
-			if (strpos($header, ':') !== false) {
+			if (str_contains($header, ':')) {
 				[$heading, $value] = explode(':', $header, 2);
 
 				if (($index = array_search($heading, $selectedHeaders, true)) !== false) {

src/Mail/MimePart.php

@@ -257,7 +257,7 @@ public function getEncodedMessage(): string
 		}
 
 		if ($this->parts) {
-			if (substr($output, -strlen(self::EOL)) !== self::EOL) {
+			if (!str_ends_with($output, self::EOL)) {
 				$output .= self::EOL;
 			}