Update Bundler to 4.0.6

This commit bumps the bundler version 4.0.6 that includes ruby/rubygems#9301
to make `bundle update <gem>` works.

- Bundler 4.0.4
```
$ ruby -v
ruby 4.0.1 (2026-01-13 revision e04267a14b) +PRISM [x86_64-linux]
$ gem -v
4.0.5
$ bundler -v
4.0.4
$ bundle update dalli
Fetching gem metadata from https://rubygems.org/.........
Resolving dependencies...
Could not find compatible versions

Because every version of releaser depends on rake ~> 13.0
  and rake ~> 13.0 could not be found in source at `tools/releaser`,
  releaser cannot be used.
So, because Gemfile depends on releaser >= 0,
  version solving has failed.
$
```

- Bundler 4.0.6

```ruby
$ gem update --system
$ bundle update --bundler
Fetching gem metadata from https://rubygems.org/.
Updating bundler to 4.0.6.
Fetching gem metadata from https://rubygems.org/.
Fetching gem metadata from https://rubygems.org/.........
Resolving dependencies...
Bundle updated!
1 installed gem you directly depend on is looking for funding.
  Run `bundle fund` for details
$ bundle update dalli
Fetching gem metadata from https://rubygems.org/.........
Resolving dependencies...
Resolving dependencies...
Using dalli 4.3.0 (was 4.0.0)
Bundle updated!
1 installed gem you directly depend on is looking for funding.
  Run `bundle fund` for details
```

Author: yahonda

Gemfile.lock

@@ -757,4 +757,4 @@ DEPENDENCIES
   websocket-client-simple
 
 BUNDLED WITH
-  4.0.4
+  4.0.6