chore: rename 'first-organization' to 'coder' (#14808)

Rename the first-organization original name. Users can change from the
original name.

coderd/database/migrations/000259_rename_first_organization.down.sql

@@ -0,0 +1,3 @@
+-- Leave the name as 'coder', there is no downside.
+-- The old name 'first-organization' is not used anywhere, just the
+-- is_default property.

coderd/database/migrations/000259_rename_first_organization.up.sql

@@ -0,0 +1,10 @@
+UPDATE
+	organizations
+SET
+	name = 'coder',
+	display_name = 'Coder'
+WHERE
+	-- The old name was too long.
+	name = 'first-organization'
+	AND is_default = true
+;