Fix: Cursor.php return type hint breaks php < 8

[sesanzb]

Fixes issue introduced in versions > 15.0.2 when current() function doc line: "* @return AbstractObject|bool" was changed by return type hint: "public function current() : AbstractObject|bool {"

Changed current() return typing from AbstractObject|bool to AbstractObject|null to better match what you would expect

[stcheng]

Hi @sesanzb thanks for submitting the pull request. We are considering dropping the support of PHP version < 8 for now. So this pull request might not be merged.

[sesanzb]

Hi @stcheng ! I understand, but in case this solution doesn't merge, it would be great to have a solution for 15.0.3 revision which were minor update and stopped working on PHP < 8, and to add a requirement in composer for 16.0.0 revision for php ^8

[stcheng]

@sesanzb right now we don't branch out older versions but only tag them. so since v16 has already released on top of the master branch, we're dropping the support since then.