chore: Prepare Scala 3 compatibility by adding -Xsource:3 compiler flag

[seakayone]

Pull Request Checklist

Task Description/Number

Issue Number: DEV-

PR Type

• build/chore: maintenance tasks (no production code change)
• docs: documentation changes (no production code change)
• feat: represents new features
• fix: represents bug fixes
• perf: performance improvements
• refactor: represents production code refactoring
• test: adding or refactoring tests (no production code change)

Basic requirements for bug fixes and features

• Tests for the changes have been added
• Docs have been added / updated

Does this PR introduce a breaking change?

• Yes

Does this PR change client-test-data?

• Yes

[codecov]

Codecov Report

Attention: 15 lines in your changes are missing coverage. Please review.

Comparison is base (3e95d71) 11.68% compared to head (e48b69d) 88.48%.
Report is 11 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #2924       +/-   ##
===========================================
+ Coverage   11.68%   88.48%   +76.80%     
===========================================
  Files         251      247        -4     
  Lines       23065    22922      -143     
===========================================
+ Hits         2695    20283    +17588     
+ Misses      20370     2639    -17731     

Files	Coverage Δ
...ebapi/src/main/scala/dsp/constants/SalsahGui.scala	95.00% <ø> (ø)
webapi/src/main/scala/dsp/errors/Errors.scala	20.00% <ø> (+15.00%)	⬆️
...ebapi/src/main/scala/dsp/valueobjects/Schema.scala	99.04% <100.00%> (+0.95%)	⬆️
webapi/src/main/scala/dsp/valueobjects/User.scala	71.11% <ø> (+2.22%)	⬆️
...main/scala/org/knora/webapi/config/AppConfig.scala	85.29% <ø> (ø)
...main/scala/org/knora/webapi/core/ActorSystem.scala	100.00% <ø> (+100.00%)	⬆️
...c/main/scala/org/knora/webapi/core/AppRouter.scala	100.00% <ø> (+100.00%)	⬆️
...c/main/scala/org/knora/webapi/core/AppServer.scala	91.30% <ø> (+91.30%)	⬆️
.../main/scala/org/knora/webapi/core/HttpServer.scala	100.00% <ø> (+100.00%)	⬆️
...ain/scala/org/knora/webapi/core/MessageRelay.scala	100.00% <ø> (ø)
... and 173 more

... and 41 files with indirect coverage changes

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