fix: always encode text as UTF-8 instead of the JVM default charset

[overcat]

No description provided.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.54%. Comparing base (0ddb287) to head (e8c4cd8).

Additional details and impacted files

@@             Coverage Diff              @@
##             master     #820      +/-   ##
============================================
- Coverage     83.55%   83.54%   -0.02%     
- Complexity     1710     1711       +1     
============================================
  Files           234      234              
  Lines          6165     6167       +2     
  Branches        699      699              
============================================
+ Hits           5151     5152       +1     
  Misses          629      629              
- Partials        385      386       +1     

Files with missing lines	Coverage Δ
src/main/java/org/stellar/sdk/Sep10Challenge.java	93.08% <100.00%> (+0.03%)	⬆️
src/main/java/org/stellar/sdk/SignerKey.java	91.11% <ø> (ø)
src/main/java/org/stellar/sdk/SorobanServer.java	81.17% <100.00%> (+0.05%)	⬆️
src/main/java/org/stellar/sdk/Util.java	82.35% <100.00%> (ø)
src/main/java/org/stellar/sdk/scval/Scv.java	98.07% <100.00%> (ø)

... and 1 file with indirect coverage changes

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