Upgrade JavaFX and dependencies

[mfl28]

• Upgrades to JavaFX 20.
• Upgrades Gradle version and dependencies.
• Adds explicit dependency of the Gradle testCompileJava task to compileSass task thereby removing a Gradle warning.

Closes #56 .

[codecov]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (a840b1c) 85.74% compared to head (97f3e0c) 85.74%.

Additional details and impacted files

@@            Coverage Diff            @@
##             master      #57   +/-   ##
=========================================
  Coverage     85.74%   85.74%           
  Complexity     1404     1404           
=========================================
  Files            92       92           
  Lines          5745     5745           
  Branches        521      521           
=========================================
  Hits           4926     4926           
  Misses          560      560           
  Partials        259      259           

Impacted Files	Coverage Δ
.../mfl28/boundingboxeditor/ui/ImageFileListView.java	88.17% <100.00%> (ø)
...om/github/mfl28/boundingboxeditor/ui/MainView.java	83.06% <100.00%> (ø)

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[sonarqubecloud]

Kudos, SonarCloud Quality Gate passed!   

0 Bugs
0 Vulnerabilities
0 Security Hotspots
0 Code Smells

100.0% Coverage
0.0% Duplication