Fix PHP 8.5 compatibility

[shu-yusa]

Issue: #242

Fix deprecations and add PHP 8.5 to the phpunit matrix.

Reproduction of warnings with tests (before fix):

% ./vendor/bin/phpunit  --display-deprecations
PHPUnit 11.5.50 by Sebastian Bergmann and contributors.

Runtime:       PHP 8.5.5
Configuration: /***/simplejwt/phpunit.xml.dist

...........................DD.......D.......DDD.....DDD........  63 / 113 ( 55%)
.............DDDDDDDDDDD.DDDDDDDDDDD..........DD..              113 / 113 (100%)

Time: 00:00.120, Memory: 12.00 MB

There was 1 PHPUnit test runner warning:

1) No code coverage driver available

--

33 tests triggered 2 PHP deprecations:

1) /***/simplejwt/src/SimpleJWT/Util/ASN1/DER.php:289
ord(): Providing a string that is not one byte long is deprecated. Use ord($str[0]) instead

Triggered by:

* SimpleJWT\Crypt\KeyManagement\RSAESTest::testRSA1_5 (10 times)
  /***/simplejwt/tests/KeyManagement/RSAESTest.php:11
...

2) /***/simplejwt/src/SimpleJWT/Keys/KeySet.php:327
Using null as an array offset is deprecated, use an empty string instead

Triggered by:

* KeySetTest::testPassword (4 times)
  /***/simplejwt/tests/KeySetTest.php:37
...

[shu-yusa]

cc: @kelvinmo