Change license to MIT

[Mistralys]

To stay in line with my other libraries like text-diff for example (see issue there: Mistralys/text-diff#2), this library should have its license switched to the MIT license.

[Mistralys]

Double-checking licenses of dependencies, and if they are MIT-compatible:

• geshi: GPL 2.0 = NO
• neitanod/forceutf8: BSD3-Clause = YES
• symfony/polyfill-php8: MIT = YES
• parsecsv/php-parsecsv: MIT = YES

To switch to MIT, this means that geshi must be replaced with another solution.

[Mistralys]

Note: An alternative to geshi could be https://github.com/scrivo/highlight.php, which is BSD-3.