Keycloak JS 26.2.1 released

This release of Keycloak JS is the first release after our [initial announcement](https://www.keycloak.org/2025/02/keycloak-js-2620-released) to split if off from the main project release cycle. This release is the result of a large internal refactor to make the code more maintainable and make use of modern JavaScript language features, as well as to introduce a new test suite with more comprehensive test coverage. Even though much has changed under the hood, this is a patch releases, and there should be no breaking changes for users, only bugfixes and small enhancements.