chore: Exclude code/unreachable transaction code from Codecov (#5847)

This change excludes from Codecov unreachable/difficult-to-test transaction code (such as `tecINTERNAL`) and old code (from amendments that have been enabled for a long time that are only around for ledger replay reasons). This removes about 200 lines of misses and increases the Codecov coverage by 0.3% (79.2% to 79.5%).

src/xrpld/app/tx/detail/MPTokenAuthorize.cpp

@@ -75,7 +75,7 @@ MPTokenAuthorize::preclaim(PreclaimContext const& ctx)
                 auto const sleMptIssuance = ctx.view.read(
                     keylet::mptIssuance(ctx.tx[sfMPTokenIssuanceID]));
                 if (!sleMptIssuance)
-                    return tefINTERNAL;
+                    return tefINTERNAL;  // LCOV_EXCL_LINE
 
                 return tecHAS_OBLIGATIONS;
             }