[UPMERGE] 2.0 -> 2.1 (#279)

This PR has been generated automatically.
For more details see
[upmerge_pr.yaml](/Sylius/MolliePlugin/blob/1.0/.github/workflows/upmerge_pr.yaml).

**Remember!** The upmerge should always be merged with using `Merge pull
request` button.

In case of conflicts, please resolve them manually with usign the
following commands:
```
git fetch upstream
gh pr checkout <this-pr-number>
git merge upstream/2.1 -m "Resolve conflicts between 2.0 and 2.1"
```

If you use other name for the upstream remote, please replace `upstream`
with the name of your remote pointing to the `Sylius/MolliePlugin`
repository.

Once the conflicts are resolved, please run `git merge --continue` and
push the changes to this PR.

Author: NoResponseMate

README.md

@@ -21,7 +21,7 @@
 ## Table of Contents
 
 * [Overview](#overview)
-* [Demo](#mollie-plugin-demo)
+* [Demo](#demo)
 * [Installation](#installation)
   * [Usage](#usage)
   * [Recurring subscription (internal CRON)](doc/recurring.md)