Update Simple XML sitemap module from ~3.0 to ~4.0

Problem/Motivation

Simple XML sitemap 4.0.0-alpha1 was released 30 Jun 2021 at 17:24 EEST
https://www.drupal.org/project/simple_sitemap/releases/4.0.0-alpha1

In the release notes by Pawel Ginalski (gbyte)

This is already very stable and I would appreciate everyone, who doesn't depend on the module API to use it immediately! For those of you who rely on the API: it may still change until the stable release.

Upgrade path
Upgrade path from 3.x and 2.x for regular users is available.

Proposed resolution

Change to ~4.0 in the composer.json file

Remaining tasks

• Change version to ~4.0
• Test on install/update
• Review
• Automated testing
• Release

User interface changes

• None

API changes

• None

Data model changes

• None