Migrate gl-text-orange-500 to warning icon variant and gl-text-warning

What does this MR do and why?

Migrates gl-text-orange-500 to warning icon variant and gl-text-warning class

References

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MR acceptance checklist

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Screenshots or screen recordings

Some examples of affected areas

Area	Screenshot
Epic boards sidebar ancestors tree
Todo due date
Confidential icon (here on issue boards)

How to set up and validate locally

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Related to #503126 (closed)