Remove unsupported Spring Boot versions from test matrix #6370

Spring Boot 2.4 is one example: it reached end of support in November 2021 and supports Java only through 18, while the build now runs on JDK 21. The sample module currently targets Java 11. Remove all unsupported Spring Boot versions from the v9 test matrix.

• Spring Boot 2.6 announcement

Action taken on behalf of Nelson Osacky.