Merge hotfix v1.3.1: allow lowercase letters in asset codes

[christian-rogobete]

Summary

Merge the v1.3.1 hotfix into main. This brings the asset code validation fix and version updates from the hotfix branch.

• Fix asset code validation to accept alphanumeric characters (a-z, A-Z, 0-9), matching JS and Python Stellar SDKs
• Remove auto-uppercase transformations from demo app input fields
• Update documentation and compatibility matrices to v1.3.1

Fixes #14

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 77.33%. Comparing base (fc55079) to head (feb07ca).
⚠️ Report is 89 commits behind head on main.

❌ Your project status has failed because the head coverage (77.33%) is below the target coverage (80.00%). You can increase the head coverage or adjust the target coverage.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #16      +/-   ##
==========================================
- Coverage   82.37%   77.33%   -5.05%     
==========================================
  Files         806      836      +30     
  Lines       20505    24024    +3519     
  Branches     2650     3173     +523     
==========================================
+ Hits        16892    18580    +1688     
- Misses       2466     4164    +1698     
- Partials     1147     1280     +133     

Files with missing lines	Coverage Δ
.../commonMain/kotlin/com/soneso/stellar/sdk/Asset.kt	84.34% <100.00%> (-0.27%)	⬇️
...k/sep/sep38/exceptions/Sep38BadRequestException.kt	100.00% <ø> (ø)

... and 52 files with indirect coverage changes

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