Requirements are inconsistent regarding Java 11

[Bananeweizen]

In #340 EclipseCS was upgraded in parts to be aligned with the Checkstyle 10.0 Java 11 requirements. Some parts of that don't look correct to me:

• the minimum required Eclipse version should be 2020-09, since that is the first to require Java 11 as runtime. See https://wiki.eclipse.org/Eclipse/Installation
• bundles should use Java 11 compiler compliance settings instead of 8 (right now, only their classpath is set to use the 11 runtime)
• the eclipse feature should require org.eclipse.core.runtime 3.19 or later (since that is the bundle version in 2020-09), otherwise the feature can still be installed into older versions and will break after restart only.