Update Java project setup

[stefanscheidt]

• Use Java 17
• Use Maven Wrapper
• Update JUnit version
• Remove ApprovalTests dependency (not used)
• improve gitignore for IntelliJ
• Minor cleanup in TennisTest.java

[emilybache]

Thankyou so much! I appreciate all the little explanatory comments in the PR. I understood what you did and it looks good.