Update Bundler to avoid permission error on GHA

pocke · pocke · commit 88aa628c6431 · 2025-01-31T19:17:01.000+09:00
The following error happens in GHA and updating Bundler will solve it.

```
The installation path is insecure. Bundler cannot continue.
```
diff --git a/Gemfile.lock b/Gemfile.lock
@@ -196,4 +196,4 @@ DEPENDENCIES
   test-unit
 
 BUNDLED WITH
-   2.5.16
+   2.6.3
diff --git a/steep/Gemfile.lock b/steep/Gemfile.lock
@@ -76,4 +76,4 @@ DEPENDENCIES
   steep (~> 1.9.2)
 
 BUNDLED WITH
-   2.5.16
+   2.6.3
