Convert from attr_encrypted to encrypted

MR: Pending

Description

Convert from attr_encrypted to encrypted to comply with Gitlab/Rails/AttrEncrypted rubocop rule. See https://docs.gitlab.com/development/migration_style_guide/#encrypted-attributes

As a [user or stakeholder], I want [goal or objective] so that [reason or benefit].

Acceptance criteria

• Convert ee/app/models/remote_development/workspace_variable.rb to use encrypted

Implementation plan

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[Provide a high-level plan for implementation of this issue, including relevant technical and/or design details.]