Update package to Laravel 9 compatibility

[ishigami]

• I've merged develop branch (outdated) with the master branch.
• Added the corcel/corcel version to ^6.0
• Added phpunit version to ^9.0
• Updated readme.md to add latest corcel version
• Updated phpunit.xml schema

PHPunit test results:

PHPUnit 9.5.16 by Sebastian Bergmann and contributors.

Runtime:       PHP 8.0.15
Configuration: /Volumes/kandisk/work/open-source-works/acf/phpunit.xml

...................................S............                  48 / 48 (100%)

Time: 00:00.209, Memory: 12.00 MB

There was 1 skipped test:

1) JqueryFieldsTests::testGoogleMapField
Google Map field i s not working at the moment

/Volumes/kandisk/work/open-source-works/acf/tests/JqueryFieldsTest.php:21

OK, but incomplete, skipped, or risky tests!
Tests: 48, Assertions: 105, Skipped: 1.

[antodd]

👍 @jgrossi . It would be great to have the latest Corcel and Laravel supported in this package.

[sietzekeuning]

can this please be merged?

[AntonCooper]

+1 for getting a release for this. Would be super helpful

[fidovdbos]

@jgrossi @jakobbuis Please release this version including the corcel 6 implementation so we can continue to use this

[georgi-vdimitrov]

+1. We still need this @jgrossi. Thanks in advance!

[XenBG]

+1

[aon4o]

+1