Hardcode invoice date

To stop it changing for all the invoices when I update a single member
count or address.

invoices/sent/2022/autonomic_co-operative_limited.html

@@ -59,7 +59,7 @@ United Kingdom
     </div>
 
     <p class="date">
-	05 May 2022
+	03 May 2022
     </p>
 
     <h1>Invoice: CoTech Fund Contribution</h1>

scripts/generate_invoices.rb

@@ -1,8 +1,8 @@
 require 'csv'
 require 'erb'
-require 'date'
 
 FIRST_INVOICE_NUMBER = 50
+INVOICE_DATE = '03 May 2022'
 
 output_dir = File.join(File.dirname(__FILE__), "..", "invoices", "sent", "2022")
 
@@ -20,7 +20,7 @@ class Invoice
   end
 
   def date
-    Date.today.strftime('%d %B %Y')
+    INVOICE_DATE
   end
 
   def number