Shorten bulk voucher default length

When bulk creating vouchers it was using a code length of 14 characters. Doing the math
shows that with just 6 characters of length you need more than 33 thousand codes in
order to have a 1% chance of collision (assuming I was using that online calculator
correctly). That's more than enough.

src/routes/admin/api/transactions.js

@@ -109,7 +109,8 @@ export async function editVoucher(id, code, amount, activated, expiresAt){
 	});
 }
 
-function generateRandomString(forbidden=new Set(), length=14, chars="ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789"){
+// >33k codes needed for a 1% chance of a collision
+function generateRandomString(forbidden=new Set(), length=6, chars="ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789"){
 	let result = "";
 	for(let i = 0; i < length; i++){
 		result += chars.charAt(Math.floor(Math.random() * chars.length));