Use Java 15-21+ features

[axlewin]

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[codecov]

Codecov Report

❌ Patch coverage is 33.46614% with 167 lines in your changes missing coverage. Please review.
✅ Project coverage is 39.85%. Comparing base (abe0f55) to head (d2b2716).
⚠️ Report is 4 commits behind head on main.

Files with missing lines	Patch %	Lines
...ava/uk/ac/cam/cl/dtg/segue/etl/ContentIndexer.java	8.69%	15 Missing and 6 partials ⚠️
...n/java/uk/ac/cam/cl/dtg/isaac/api/PagesFacade.java	11.76%	14 Missing and 1 partial ⚠️
...in/java/uk/ac/cam/cl/dtg/isaac/api/QuizFacade.java	23.07%	9 Missing and 1 partial ⚠️
.../ac/cam/cl/dtg/isaac/api/managers/GameManager.java	0.00%	6 Missing and 4 partials ⚠️
...cam/cl/dtg/segue/api/managers/QuestionManager.java	10.00%	5 Missing and 4 partials ⚠️
.../ac/cam/cl/dtg/isaac/api/managers/QuizManager.java	37.50%	4 Missing and 1 partial ⚠️
...cam/cl/dtg/segue/search/ElasticSearchProvider.java	72.22%	1 Missing and 4 partials ⚠️
...c/cam/cl/dtg/isaac/api/managers/EventsManager.java	0.00%	2 Missing and 2 partials ⚠️
...cl/dtg/isaac/api/managers/QuizQuestionManager.java	20.00%	4 Missing ⚠️
...tg/isaac/quiz/IsaacSymbolicChemistryValidator.java	0.00%	4 Missing ⚠️
... and 56 more

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #768      +/-   ##
==========================================
- Coverage   39.87%   39.85%   -0.03%     
==========================================
  Files         547      547              
  Lines       23909    23741     -168     
  Branches     2873     2873              
==========================================
- Hits         9533     9461      -72     
+ Misses      13470    13374      -96     
  Partials      906      906              

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