StateTimeline: Add gradient to value disconnect

[drew08t]

For ranges where values have been disconnected, apply a transparent (fall off) gradient styling to indicate uncertainty within that region. This will be applied up until the threshold time, at which case it will be completely transparent until the next state.

Also see addressing strange rendering behavior pointed out in: #70610 (comment)

[grafanabot]

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[leeoniya]

wonder if the hover tooltip for these regions should also somehow indicate that the duration is approximate due to implicit cutoff? 🤔

[nmarrs]

If this is still around and not addressed by Q4 2023 consider closing