testnet update: rename testnet3 to testnet

Update all references to 'testnet3' to 'testnet' for consistency. This includes:
- Changing magic bytes in network messages
- Updating directory paths and configuration files
- Modifying documentation and test framework references
- Adjusting chain parameters and validation logic

src/validation.cpp

@@ -2039,7 +2039,7 @@ bool CChainState::ConnectBlock(const CBlock& block, BlockValidationState& state,
     // edge case when manipulating the UTXO and it would be simpler not to have
     // another edge case to deal with.
 
-    // testnet3 has no blocks before the BIP34 height with indicated heights
+    // testnet has no blocks before the BIP34 height with indicated heights
     // post BIP34 before approximately height 486,000,000 and presumably will
     // be reset before it reaches block 1,983,702 and starts doing unnecessary
     // BIP30 checking again.