Adjust php-stubs/wordpress-stubs dependency range so 7.0 can be used

[justlevine]

Allows this to be used alongside the latest WordPress stubs.

Your requirements could not be resolved to an installable set of packages.

  Problem 1
    - Root composer.json requires php-stubs/wp-cli-stubs ^2.12 -> satisfiable by php-stubs/wp-cli-stubs[v2.12.0].
    - php-stubs/wp-cli-stubs v2.12.0 requires php-stubs/wordpress-stubs ^4.7 || ^5.0 || ^6.0 -> found php-stubs/wordpress-stubs[v4.7.14, ..., v4.9.20, v5.0, ..., v5.9.9, v6.0.0, ..., v6.9.4] but it conflicts with your root composer.json require (~7.0

Opted for >= from szepeviktor/phpstan-wordpress#307 , instead of appending || ^7.0

[szepeviktor]

Here you go!