fix(GlLabel): restore text-decoration underline on hover for labels

What does this MR do?

Add gl-label-link-underline class fo text-decoration: underline on inner gl-label-text and gl-label-text-scoped elements so that they inherit relative color. This ensures scoped labels will have underline colors that match the text color.

Discussed in !4103 (comment 1993500972) and !4279 (comment 1989332161)

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