feat(abstract-utxo): enable legacy tx format for mainnet

Changes the supportedTxFormats property from readonly to mutable
and sets legacy format support based on network type. Legacy
format is now enabled for mainnet networks while remaining
disabled for testnet.

Issue: BTC-0

Co-authored-by: llm-git <llm-git@ttll.de>

Author: OttoAllmendinger

modules/abstract-utxo/src/abstractUtxoCoin.ts

@@ -421,9 +421,9 @@ export abstract class AbstractUtxoCoin
 
   public readonly amountType: 'number' | 'bigint';
 
-  protected readonly supportedTxFormats: { readonly psbt: boolean; readonly legacy: boolean } = {
+  protected supportedTxFormats: { psbt: boolean; legacy: boolean } = {
     psbt: true,
-    legacy: false,
+    legacy: this.isMainnet(),
   };
 
   protected constructor(bitgo: BitGoBase, amountType: 'number' | 'bigint' = 'number') {