for merged platforms, date of sequence update should reflect the parent platform

[ppavlidis]

On this page https://gemma.msl.ubc.ca/arrays/showAllArrayDesigns.html

The dates shown for merged and subsumed platforms are often incorrect, at least for the sequence analysis. What is being shown is the date of update of the merged platform, and we no longer do direct updates of merged platforms.

Instead, the update date should be pulled for the "parent" platform, unless there was direct processing of the platform (for some reason) that would have a more recent date.