Fix Arel deprecations for Rails 6

What does this MR do?

Strings passed to #order or #pluck are now required to be wrapped in Arel.sql. Some simple expressions like column names and functions with at most 1 argument are whitelisted in https://github.com/rails/rails/pull/36448 which will be in Rails 6 so I did not change these here.

This also fixes a deprecation regarding delegating arel methods automatically. Calling arel explicitly is now required.

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
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• Style guides
• Database guides
• Separation of EE specific content

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• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers

Security

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Closes #28497 (closed)