Fix build compatibility for OMNeT++ 6.1, 6.2, and 6.3+

[tanapatnamsomboon]

This PR fixes compilation errors caused by API changes in recent OMNeT++ versions.

I have verified that the project builds and runs successfully on OMNeT++ 6.1.0, 6.2.0, and 6.3.0 with quisp_tutorial.

Key Changes:

1. quisp/modules/Logger/JsonLogger.cc:

   • Explicitly called .str() on simTime() to fix formatting issues with strict type checking in newer compilers/OMNeT++ versions. This change is backward compatible.

2. quisp/test_utils/ModuleType.h:

   • Implemented getDocumentation() override. This method became a required pure virtual function starting from OMNeT++ 6.2.
   • Guarded with #if OMNETPP_VERSION >= 0x602.

3. quisp/test_utils/StaticEnv.h:

   • Updated registerOutputVector signature (now requires 3 arguments in v6.1+).
   • Handled setVectorAttribute (removed override for v6.1+ as the virtual method signature changed/removed).
   • Implemented getOutputStream (required pure virtual method in v6.x).
   • All changes are guarded with #if OMNETPP_VERSION >= 0x601.

Test Environment:

• OS: Linux Ubuntu 24.04 LTS
• Compiler: Clang 18.1.3 (Build verified), GCC 13.3.0
• OMNeT++ Versions Verified: 6.1.0, 6.2.0, 6.3.0

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This change is