Drupal 9 compatibility

Problem/Motivation

Drupal 9 is here (9.0.3 already) and this module has about nothign to do to be compatible according to Drupal Rector.

$ vendor/bin/rector process web/modules/contrib/migrate_spreadsheet --dry-run
Rector v0.7.29
Config file: rector.yml

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 [OK] Rector is done!

The thing is that we cannot easily fake composer to install this module on D9 so we'd need it to be marked as compatible.
Being compatible with D9 bumps the php requirement to 7+ so we need to tag a new major version to prevent old websites to upgrade by mistake.

Proposed resolution

1. Change the migrate_spreadsheet.info.yml file to remove the PHP version and add the new core requirement key.
2. Tag a new 2.0.0 major release.