feat: notify users on template deprecation (#15195)

Closes https://github.com/coder/coder/issues/15117

Notify users when a template has been deprecated.

coderd/database/migrations/000270_template_deprecation_notification.down.sql

@@ -0,0 +1 @@
+DELETE FROM notification_templates WHERE id = 'f40fae84-55a2-42cd-99fa-b41c1ca64894';

coderd/database/migrations/000270_template_deprecation_notification.up.sql

@@ -0,0 +1,22 @@
+INSERT INTO notification_templates
+	(id, name, title_template, body_template, "group", actions)
+VALUES (
+	'f40fae84-55a2-42cd-99fa-b41c1ca64894',
+	'Template Deprecated',
+	E'Template ''{{.Labels.template}}'' has been deprecated',
+    E'Hello {{.UserName}},\n\n'||
+		E'The template **{{.Labels.template}}** has been deprecated with the following message:\n\n' ||
+		E'**{{.Labels.message}}**\n\n' ||
+		E'New workspaces may not be created from this template. Existing workspaces will continue to function normally.',
+    'Template Events',
+	'[
+		{
+			"label": "See affected workspaces",
+			"url": "{{base_url}}/workspaces?filter=owner%3Ame+template%3A{{.Labels.template}}"
+		},
+		{
+			"label": "View template",
+			"url": "{{base_url}}/templates/{{.Labels.organization}}/{{.Labels.template}}"
+		}
+	]'::jsonb
+);