Update required nokogiri version

The newest version 1.18.9 provides fixes for several libxml2 CVEs which
is vendored in nokogiri.

Author: zdohnal

Gemfile.lock

@@ -257,14 +257,14 @@ GEM
     mutex_m (0.3.0)
     net-http (0.6.0)
       uri
-    nokogiri (1.18.8)
+    nokogiri (1.18.9)
       mini_portile2 (~> 2.8.2)
       racc (~> 1.4)
-    nokogiri (1.18.8-arm64-darwin)
+    nokogiri (1.18.9-arm64-darwin)
       racc (~> 1.4)
-    nokogiri (1.18.8-x86_64-darwin)
+    nokogiri (1.18.9-x86_64-darwin)
       racc (~> 1.4)
-    nokogiri (1.18.8-x86_64-linux-gnu)
+    nokogiri (1.18.9-x86_64-linux-gnu)
       racc (~> 1.4)
     octokit (4.25.1)
       faraday (>= 1, < 3)