Raise version to 2.3.1

Description

Raises the version number to 2.3.1.

Changes

• Changes the version to 2.3.1.

Additions

None.

Test and Review

To be filled in by the reviewers

• There are no unnecessary files present in the MR

• The continuous integration has no problems with the MR

• The MR is filled in as requested (including labels, milestones, and reviewers)

• The documentation is up-to-date