Use Java 17 instead of 16 in build

[beatngu13]

Closes #521.

Gradle noob question: as described here, I ran the following command twice:

$ ./gradlew wrapper --gradle-version 7.3

The gradle-wrapper.jar and gradle executables, however, didn't change. Did I do something wrong?

Anyway, this PR should be merged after #535.

Proposed commit message:

Use Java 17 instead of 16 in build (#521 / #536)

Since this requires Gradle 7.3, this PR updates the Gradle wrapper
accordingly.

Closes: #521
PR: #536

---

PR checklist

The following checklist shall help the PR's author, the reviewers and maintainers to ensure the quality of this project.
It is based on our contributors guidelines, especially the "writing code" section.
It shall help to check for completion of the listed points.
If a point does not apply to the given PR's changes, the corresponding entry can be simply marked as done.

Documentation (general)

• There is documentation (Javadoc and site documentation; added or updated)
• There is implementation information to describe why a non-obvious source code / solution got implemented
• Site documentation has its own .adoc file in the docs folder, e.g. docs/report-entries.adoc
• Only one sentence per line (especially in .adoc files)
• Javadoc uses formal style, while sites documentation may use informal style

Documentation (new extension)

• The docs/docs-nav.yml navigation has an entry for the new extension
• The package-info.java contains information about the new extension

Code

• Code adheres to code style, naming conventions etc.
• Successful tests cover all changes
• There are checks which validate correct / false usage / configuration of a functionality and there are tests to verify those checks
• Tests use AssertJ or our own PioneerAssert (which are based on AssertJ)

Contributing

• A prepared commit message exists
• The list of contributions inside README.md mentions the new contribution (real name optional)

---

I hereby agree to the terms of the JUnit Pioneer Contributor License Agreement.

[aepfli]

lgtm ;)

[aepfli]

the executable and the jar sometimes simply do not change - the important part is, that the script and the jar download the new gradle version, and this is donewith this url change