Allow changelog plugin to be run in dry-run mode in MR revert

What does this MR do and why?

Previously, the changelog plugin could not handle the following case:

1. Danger is run in dry-run mode (no API token)
2. Danger is run in a MR revert

It always failed because it was unable to fetch the current milestone and MR labels.

To side-step this issue we allow revert MRs to pass this change in this case.

Refs gitlab-org/gitlab#466701 (comment 1946291381)

How to verify on CI

1. Create a single commit with the message Revert "me"
2. Ensure that a required reason is triggered changelog (e.g. feature_flag removed)

diff --git a/lib/danger/plugins/changelog.rb b/lib/danger/plugins/changelog.rb
index d485eeb..5aab240 100644
--- a/lib/danger/plugins/changelog.rb
+++ b/lib/danger/plugins/changelog.rb
@@ -208,7 +208,7 @@ module Danger
     def required_reasons
       [].tap do |reasons|
         reasons << :db_changes if helper.changes.added.has_category?(:migration)
-        reasons << :feature_flag_removed if helper.changes.deleted.has_category?(:feature_flag)
+        reasons << :feature_flag_removed # if helper.changes.deleted.has_category?(:feature_flag)
       end
     end
 

3. Commit and push
4. Check output of danger-review and danger-review dry-run CI jobs
5. Apply regression:17.1 (current milestone) and re-run CI jobs

Before	Before with regression	After	After with regression
❌ danger-review ❌ danger-review dry-run	✅ danger-review ❌ danger-review dry-run	❌ dange-review ✅ danger-review dry-run	✅ danger-review ✅ danger-review dry-run