Fixed type-incorrect Gson configuration methods

[rodinaarssen]

This caused UI tests to fail in rascal-lsp's CI.

[codecov]

Codecov Report

❌ Patch coverage is 0% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 46%. Comparing base (51be61a) to head (1a2878f).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/org/rascalmpl/ideservices/GsonUtils.java	0%	2 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##              main   #2619   +/-   ##
=======================================
  Coverage       46%     46%           
- Complexity    6647    6648    +1     
=======================================
  Files          793     793           
  Lines        65719   65719           
  Branches      9844    9844           
=======================================
+ Hits         30597   30612   +15     
+ Misses       32755   32746    -9     
+ Partials      2367    2361    -6     

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