Update introduction for Java 21

[logological]

The current version of DKPro Core requires Java 8 or higher, but the Intro using Java page instructs users to install Java 7. This can cause the following problems:

• The Oracle page linked to from the instructions no longer offers Java 7 for download.
• Many operating systems no longer package a Java 7 SDK.
• Even if users already have or somehow manage to get a Java 7 SDK installed in order to do the tutorial, they'll need to upgrade to Java 8 or higher anyway if they want to go on to use the current version of DKPro Core in their own projects.

The tutorial should be changed to use Java 17 or higher.

[reckart]

Should go Java 11 now.

[reckart]

Bumped to Java 17 cough