refactor(target-postgres): rename _ctx to ctx in resolveDropConstraint (it is used)

The _ prefix convention signals an unused parameter, but resolveDropConstraint
reads ctx.schemaName. Rename the parameter + usage site to match the
convention used elsewhere in this file.

Author: wmadden

packages/3-targets/3-targets/postgres/src/core/migrations/operation-resolver.ts

@@ -256,9 +256,9 @@ function resolveAddForeignKey(
 
 function resolveDropConstraint(
   desc: DropConstraintDescriptor,
-  _ctx: OperationResolverContext,
+  ctx: OperationResolverContext,
 ): ResolvedOp {
-  return createDropConstraint(_ctx.schemaName, desc.table, desc.constraintName);
+  return createDropConstraint(ctx.schemaName, desc.table, desc.constraintName);
 }
 
 function resolveCreateIndex(