Remove Java 8 from GitHub workflow (ECJ needs Java 11+)

Remove Java 8 from GitHub workflow (ECJ needs Java 11+)

Since JDT Core 3.27 (Java 17) and Eclipse 2021-09 (4.21), respectively,
ECJ no longer works on JDK 8. Even if we backport JDT Core classes, some
of its dependencies contain Java 11 class files, which ultimately also
means that ACJ no longer works below JDK 11 due to those transitive
dependencies, e.g. org.eclipse.core.resources-3.14.0.jar.

For now, I added JDK 14 to the build matrix, i.e. we currently have 11,
14, 17. When JDK 18 is released, we can switch to 11, 17, 18, i.e. keep
the two LTS releases plus the latest one.

Signed-off-by: Alexander Kriegisch <Alexander@Kriegisch.name>