Upgrade rack to fix "assigned but unused variable"

Previously, using Rack::Sendfile would result in the following warning:

```
/usr/local/bundle/gems/rack-3.0.7/lib/rack/sendfile.rb:114: warning: assigned but unused variable - status
```

and this would print more than 150 times when running
test/application/configuration_test.rb in Railties. This commit fixes
the warnings by updating rack to 3.0.8 where the warning was fixed.

Gemfile.lock

@@ -371,7 +371,7 @@ GEM
       pg (>= 1.1, < 2.0)
     raabro (1.4.0)
     racc (1.6.2)
-    rack (3.0.7)
+    rack (3.0.8)
     rack-cache (1.13.0)
       rack (>= 0.4)
     rack-session (2.0.0)