Update minimum requirement PHP 7.0 to 7.2

[SunMar]

Hi,

Today I happen to notice that Maatwebsite/Laravel-Excel still says in the composer.json to be compatible with PHP 7.0. However you depend on PhpSpreadsheet and they already switched to 7.1+ in 1.9.0 and to 7.2+ in 1.13.0. That means that indirectly Laravel-Excel has already been requiring PHP 7.1+ since 3.1.18 and PHP 7.2+ since 3.1.20 because of the minimum version requirements on PhpSpreadsheet. Don't know if there are pipelines or compatibility checks still being done against PHP 7.0 and 7.1 but if so they could be removed and composer.json updated to require 7.2+.

[patrickbrouwers]

We haven't supported 7.0 anymore for a long while, so no pipelines are being ran against those versions.