Release v5.9 preparation - set "GitVersion" explicitely

[koppor]

We set the new major version to 6 directly after the last release (e586da3). However, we decided to release version 5.9 inbetween. Thus, we need to rool back.

Possibilities:

• Open a separate branch, cherry-pick all commits but leave out the semver commit. -- I tried with git rebase -i v5.8, but that was not good
• Manually set the version

This PR implements the second approach. Note that this will NOT re-open #5399 and #5657 (linked from
#5682), because the patch number is increased by using the number of commits since the last (hard-coded) release v5.8.

Resulting about dialog:

• Change in CHANGELOG.md described in a way that is understandable for the average user (if applicable)
• Tests created for changes (if applicable)
• Manually tested changed features in running JabRef (always required)
• Screenshots added in PR description (for UI changes)
• Checked developer's documentation: Is the information available and up to date? If not, I outlined it in this pull request.
• Checked documentation: Is the information available and up to date? If not, I created an issue at https://github.com/JabRef/user-documentation/issues or, even better, I submitted a pull request to the documentation repository.

[calixtus]

changelog needs to be adapted too