Drop PHP < 8.1 Support

Drop support for PHP < 8.1.

1.3

What's Changed

• Array keys and new method by @defro in #7
• Korean update by @adammeyer in #8
• add ISO639::code2tByCode1 method by @cHoXu in #9
• Fix Test for Korean by @matriphe in #11

New Contributors

• @defro made their first contribution in #7
• @adammeyer made their first contribution in #8
• @cHoXu made their first contribution in #9
• @matriphe made their first contribution in #11

Full Changelog: 1.2...1.3