Update shoulda-matchers to version 4.0.1

We use custom validation error messages for some of our models. With the
new gem release the error message shoulda matchers would expect stopped
matching our custom messages and the tests failed.
To solve this we explicitly set the validation message where needed.

The "optional: false" parameter for the belongs_to association was
superfluous since Rails uses "required" by default and caused some a
shoulda matcher to fail.

Co-authored-by: Björn Geuken <bgeuken@suse.de>