Fix deprecation warning in ActiveModel::Errors

What does this MR do?

Fix deprecation warning in ActiveModel::Errors.

From the deprecation warning:

DEPRECATION WARNING: Enumerating ActiveModel::Errors as a hash has been deprecated. In Rails 6.1, errors is an array of Error objects, therefore it should be accessed by a block with a single block parameter like this:

person.errors.each do |error|
  attribute = error.attribute
  message = error.message
end

You are passing a block expecting two parameters, so the old hash behavior is simulated. As this is deprecated, this will result in an ArgumentError in Rails 6.2. (called from map at /Users/tanle/Code/gitlab/gitlab-development-kit/gitlab/app/helpers/form_helper.rb:13)

🎀 Bonuses

• Fix Rails/ContentTag violations
• Fix duplicate method calls

Steps to reproduce

Run the following spec and observe the above deprecation warning.

bundle exec rspec --format documentation gitlab/spec/helpers/form_helper_spec.rb

Does this MR meet the acceptance criteria?

Conformity

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• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.
• This change is backwards compatible across updates, or this does not apply.

Availability and Testing

• I have added/updated tests following the Testing Guide, or it's not needed. (Consider all test levels. See the Test Planning Process.)
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