feat(coderd): remove greetings from notifications templates (#16991)

This PR aimes to [fix this
issue](https://github.com/coder/internal/issues/448) -

The main idea is to remove greetings from templates stored in the DB -
and instead push it into the template for require methods - for now
SMTP.

coderd/notifications/dispatch/smtp/html.gotmpl

@@ -14,6 +14,7 @@
         {{ .Labels._subject }}
       </h1>
       <div style="line-height: 1.5;">
+        <p>Hi {{ .UserName }},</p>
         {{ .Labels._body }}
       </div>
       <div style="text-align: center; margin-top: 32px;">

coderd/notifications/dispatch/smtp/plaintext.gotmpl

@@ -1,3 +1,5 @@
+Hi {{ .UserName }},
+
 {{ .Labels._body }}
 
 {{ range $action := .Actions }}