Add support for webform 6.x branch

Problem/Motivation

Wanted to install this module on a project that currently has webform 6.0.0 installed as part of the build

when trying to add the module composer returned the following error:-
Your requirements could not be resolved to an installable set of packages.

Problem 1
- Can only install one of: drupal/webform[5.x-dev, 6.0.0].
- Can only install one of: drupal/webform[6.0.0, 5.x-dev].
- drupal/govuk_webform_elements 1.2.0 requires drupal/webform ^5.0 -> satisfiable by drupal/webform[5.x-dev].
- Installation request for drupal/govuk_webform_elements 1.2 -> satisfiable by drupal/govuk_webform_elements[1.2.0].
- Installation request for drupal/webform 6.0.0 -> satisfiable by drupal/webform[6.0.0].

Steps to reproduce

add drupal/govuk_webform_elements to a drupal project with drupal/webform:^6.0 in the composer build

Proposed resolution

add webform 6.x as meeting the modules dependency requirements