drop use of unsupported_reason_add

[kbrock]

part of:

• Drop Supports' unsupports variable manageiq#22898

Deprecating unsupported_reason_add. Returning the string instead

Note, these are all the same:

unsupported_reason_add(:feature, unsupported_reason(:feature2)) unless supports_feature2?
unsupported_reason_add(:feature, unsupported_reason(:feature2)) unless supports?(:feature2)
unsupported_reason(:feature2) unless supports?(:feature2)
unsupported_reason(:feature2)

[miq-bot]

Checked commit kbrock@e4b9931 with ruby 2.7.8, rubocop 1.56.3, haml-lint 0.51.0, and yamllint
4 files checked, 0 offenses detected
Everything looks fine. 🏆