Upgrade dependencies

[mfl28]

• Upgrades dependencies (including Gradle, JavaFX).
• Upgrades Java toolchain version to latest LTS version 21.
• Fixes Gradle deprecation- and build and test dependency warnings.
• Upgrade Java version to be used in Github Actions.

Closes #133.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 86.44%. Comparing base (bfe45a8) to head (57fa482).

Additional details and impacted files

@@             Coverage Diff              @@
##             master     #134      +/-   ##
============================================
+ Coverage     86.38%   86.44%   +0.05%     
  Complexity     1491     1491              
============================================
  Files            96       96              
  Lines          5978     5974       -4     
  Branches        544      544              
============================================
  Hits           5164     5164              
+ Misses          535      531       -4     
  Partials        279      279              

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