Programming exercises: Update dependencies for Java blackbox exercise template

[b-fein]

Checklist

General

• I tested all changes and their related features with all corresponding user types on a test server.
• This is a small issue that I tested locally and was confirmed by another developer on a test server.
• Language: I followed the guidelines for inclusive, diversity-sensitive, and appreciative language.
• I chose a title conforming to the naming conventions for pull requests.

Changes affecting Programming Exercises

• I tested all changes and their related features with all corresponding user types on Test Server 2 (Jenkins and Gitlab).

Motivation and Context

In the last update #7127 we forgot to update the Java version in the pom.xml of the tests repository.

Description

Updates the Java version to 21 to match the template solution and exercise repositories.
Uses this occasion to also update plugins specified in the pom.xml.

Steps for Testing

Prerequisites:

• 1 Instructor

1. Create a new Java programming exercise of type Dejagnu/Blackbox.
2. The solution build passes all tests, the template build none.

Review Progress

Code Review

• Code Review 1
• Code Review 2

Manual Tests

• Test 1
• Test 2

[Strohgelaender]

Do you provide these dependencies within your docker image? If yes, we should probably update the image version as well.

[b-fein]

No, there is no caching in the image. No need to update it.

[Strohgelaender]

code

[Hialus]

Changes look good.

[DominikRemo]

Tested on ts2. Works as expected.

Note: One test fails for the solution build. It's unrelated to this PR but we should at least write an Issue once this PR gets merged. Edit: fixed in #7365