feat: use Java 11 as compiler target

[kschrab]

Description

Due to the increase of components which are compiled with JDK version 11, we finally take the step and migrate to Java 11, too. Since we were already compatible to Java 11 during runtime, no code had to be adjusted. From now on, we furthermore can take advantage of new Java 11 features, such as var keyword for local variables or additional String methods.

Issue(s) related to this PR

• Resolves internal issue 640

Affected parts of the online documentation

There are many parts in our online documentation which need to be adjusted with the next release.

Definition of Done

Prerequisites

• You have read CONTRIBUTING.md carefully.
• You have signed the Contributor License Agreement.
• Your GitHub user id is linked with your Eclipse Account.

Required

• The title of this merge request follows the scheme type(scope): description (in the style of Conventional Commits)
• You have assigned a suitable label to this pull request (e.g., enhancement, or bugfix)
• origin/main has been merged into your Fork.
• Coding guidelines have been followed (see CONTRIBUTING.md).
• All checks on GitHub pass.
• All tests on Jenkins pass.

Requested (can be enforced by maintainers)

• New functionality is covered by unit tests or integration tests. Code coverage must not decrease.
• If a bug has been fixed, a new unit test has been written (beforehand) to prove misbehavior
• There are no new SpotBugs warnings.

Special notes to reviewer

[schwepmo]

Since everything works on Jenkins and the changes are really minor, I think we can merge this.
We should however create an issue for the changes in the documentation.