Update to Java 16

[TheBusyBiscuit]

Description

This PR updates Slimefun to Java 16.
This is a breaking update for any servers that run older versions of Java.
Please update your Java version immediately!

This PR is still in progress. There are a lot of advantages that come with the usage of Java 16 and it would be nice to take advantage of most if not all of them.

I compiled a list of relatively easy to implement patterns here:
https://sonarcloud.io/project/issues?resolved=false&rules=java%3AS5194%2Cjava%3AS6201%2Cjava%3AS6204%2Cjava%3AS6206&types=CODE_SMELL&id=Slimefun_Slimefun4

Anyone can contribute!
Feel free to PR to this branch with a few changes from the list above ❤️

The following newly available patterns are desirable:

"instanceof + cast"

  if (x instanceof String string) {
    string.something()
  }
  
  // instead of:

  if (o instanceof String) {
    String string = (String) o;
    string.something()
  }

switch expressions

  return switch (x) {
    case 1 -> 1;
    case 2 -> 2;
  }

  // instead of:

  switch (x) {
    case 1:
      return 1;
    case 2:
      return 2;
  }

and various other patterns, check the sonarcloud link for more details.
Of course any other sonarcloud issue is always up for grabs too!

Checklist

• I have fully tested the proposed changes and promise that they will not break everything into chaos.
• I have also tested the proposed changes in combination with various popular addons and can confirm my changes do not break them.
• I followed the existing code standards and didn't mess up the formatting.
• I did my best to add documentation to any public classes or methods I added.
• I have added Nonnull and Nullable annotations to my methods to indicate their behaviour for null values
• I added sufficient Unit Tests to cover my code.

[github-actions]

Pro Tip!
You can help us label your Pull Requests by using the following branch naming convention next time you create a pull request. ❤️

Branch naming convention	Label
feature/**	🎈 Feature
fix/**	✨ Fix
chore/**	🧹 Chores
api/**	🔧 API
performance/**	💡 Performance Optimization
compatibility/**	🤝 Compatibility

---

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[Sefiraat]

FYI, I am happy to do those stream changes noted in the sonar list but I can't be entirely sure when a collection is required as immutable or not. While I could go through and check every usage I feel that may be better served by those who know the code base far better than myself.

[Sefiraat]

Just one comment on the Chest patterns but even then it's still fine. So LGTM

[TheBusyBiscuit]

@WalshyDev Do you perhaps have time to give this a look? 👀

[sonarcloud]

Kudos, SonarCloud Quality Gate passed!   

0 Bugs
0 Vulnerabilities
0 Security Hotspots
16 Code Smells

16.8% Coverage
4.7% Duplication

[Sfiguz7]

LGTM

[Sefiraat]

👍