Exactly what it says in the title. With most JavaEE runtimes having now migrated the servlet functionality (among other things) from javax.servlet to jakarta.servlet, Spring is no exception and requires the new namespace since Version 6. Version 5 is EOL. The Spring Boot Starter provided by this project still uses the javax workspace, and is therefore broken in any reasonably up-to-date environment.
The code MUST be changed to use the new namespace for the library to continue being usable. A notice of this major breaking change should be made.