PHP 8 support

Problem/Motivation

I've tried to add this module to my site, but failed as some of it dependencies reject php 8.
What can we do to make it work with php 8?

Steps to reproduce

$ composer require 'drupal/migrate_spreadsheet:^2.1'

./composer.json has been updated
Running composer update drupal/migrate_spreadsheet
Loading composer repositories with package information
Updating dependencies
Your requirements could not be resolved to an installable set of packages.

  Problem 1
    - phpoffice/phpspreadsheet[1.0.0, ..., 1.8.2] require php ^5.6|^7.0 -> your php version (8.1.2) does not satisfy that requirement.
    - phpoffice/phpspreadsheet[1.9.0, ..., 1.12.0] require php ^7.1 -> your php version (8.1.2) does not satisfy that requirement.
    - phpoffice/phpspreadsheet[1.13.0, ..., 1.14.1] require php ^7.2 -> your php version (8.1.2) does not satisfy that requirement.
    - drupal/migrate_spreadsheet 2.1.0 requires phpoffice/phpspreadsheet ^1.0 -> satisfiable by phpoffice/phpspreadsheet[1.0.0, ..., 1.29.0].
    - Root composer.json requires drupal/migrate_spreadsheet ^2.1 -> satisfiable by drupal/migrate_spreadsheet[2.1.0].
...