Reduce emails-on-push HTML size by using a simple monospace font

With this simple change we reduce the size dramatically by reducing every instance of this:

font-family: 'Menlo', 'Liberation Mono', 'Consolas', 'DejaVu Sans Mono', 'Ubuntu Mono', 'Courier New', 'andale mono', 'lucida console', monospace"

to this:

font-family: monospace

Helps #15178

CHANGELOG-EE

@@ -7,6 +7,7 @@ v 8.8.0 (unreleased)
   - [Elastic] Add rake task for removing all indexes
   - [Elastic] Add rake task for clearing indexing status
   - Set KRB5 as default clone protocol when Kerberos is enabled and user is logged in (Borja Aparicio)
+  - Reduce emails-on-push HTML size by using a simple monospace font
 
 v 8.7.2
   - Fix MR notifications for slack and hipchat when approvals are fullfiled. !325

app/assets/stylesheets/mailers/repository_push_email.scss

@@ -2,7 +2,7 @@
 
 table.code {
   width: 100%;
-  font-family: $monospace_font;
+  font-family: monospace;
   border: none;
   border-collapse: separate;
   margin: 0;
@@ -13,7 +13,7 @@ table.code {
 
   td {
     line-height: $code_line_height;
-    font-family: $monospace_font;
+    font-family: monospace;
     font-size: $code_font_size;
   }