Update dependency rack-attack to v6.8.0 (mastodon#36471)

Co-authored-by: renovate[bot] <29139614+renovate[bot]@users.noreply.github.com>

Author: renovate[bot]

Gemfile.lock

@@ -621,7 +621,7 @@ GEM
     raabro (1.4.0)
     racc (1.8.1)
     rack (3.2.3)
-    rack-attack (6.7.0)
+    rack-attack (6.8.0)
       rack (>= 1.0, < 4)
     rack-cors (3.0.0)
       logger