fix: use StandardCharsets.UTF_8 for byte array to string conversions

[overcat]

No description provided.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81.04%. Comparing base (3b6ed46) to head (015ad42).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff              @@
##             master     #756      +/-   ##
============================================
+ Coverage     81.03%   81.04%   +0.01%     
  Complexity     1400     1400              
============================================
  Files           216      216              
  Lines          5377     5382       +5     
  Branches        529      529              
============================================
+ Hits           4357     4362       +5     
  Misses          717      717              
  Partials        303      303              

Files with missing lines	Coverage Δ
src/main/java/org/stellar/sdk/Sep10Challenge.java	93.05% <100.00%> (+0.03%)	⬆️
src/main/java/org/stellar/sdk/Sep45Challenge.java	78.34% <100.00%> (ø)
src/main/java/org/stellar/sdk/Util.java	75.51% <100.00%> (ø)
...rg/stellar/sdk/operations/AllowTrustOperation.java	91.48% <100.00%> (+0.79%)	⬆️

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