AO3-6240 Use i18n-tasks to rename numbered mailer translation keys

[Bilka2]

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• Have you added tests for any changed functionality?
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Issue

https://otwarchive.atlassian.net/browse/AO3-6240

Purpose

Rename numbered locale keys used by the mailers.

Testing Instructions

See comment on Jira issue.

References

#4497 for moving instructions, will add them to the wiki once this PR is reviewed.

Credit

Bilka (he/him)

[Bilka2]

Duplicate (copy and rename) locale key:

• Uncomment the data write config in config/i18n-tasks.yml line 25
• Comment the data read config in config/i18n-tasks.yml line 18
• Run bundle exec i18n-tasks data -f yaml | bundle exec i18n-tasks tree-mv -a 'user_mailer.change_email.changed.html' 'user_mailer.change_email.changed.text' -f yaml | bundle exec i18n-tasks data-merge
• Undo the changes to config/i18n-tasks.yml
• Manually duplicate the English key

Update any relevant views.

[sarken]

Awesome, thank you so much!