* added GitLab Perun group repository for the perun-openapi dependency
* configured local maven settings for authorization to the repo
* use personal access token in local-settings.xml to access repo locally
* use CI token in ci-settings.xml for work with CI
* upgraded to Spring boot 3.1.4 to allow compiling with Java 21, there should be no major changes
* updated README.md with instructions regarding the local access token