DRb has been extracted from stdlib to a gem

Add DRb to the Gemfile as a gem as it has been extracted from stdlib to
a standalone gem.

```
lib/miq_automation_engine/miq_ae_exception.rb:1: warning: lib/ruby/3.3.0/drb.rb was loaded from the standard library, but will no longer be part of the default gems starting from Ruby 3.4.0.
You can add drb to your Gemfile or gemspec to silence this warning.
```

Gemfile

@@ -38,6 +38,7 @@ gem "connection_pool",                                       :require => false #
 gem "dalli",                            "~>3.2.3",           :require => false
 gem "default_value_for",                "~>4.0"
 gem "docker-api",                       "~>1.33.6",          :require => false
+gem "drb",                              "~>2.1.1",           :require => false
 gem "elif",                             "=0.1.0",            :require => false
 gem "fast_gettext",                     "~>3.1"
 gem "ffi",                              "< 1.17.0",          :require => false