Extend metric expiration for Platform.Crouton.Started

We (crouton maintainers) still look at this metric occasionally.

Bug: 1242515
Change-Id: Ie66a59127c8aec4141b3ab90f6392fc3aaaf2fb7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3140511


Reviewed-by: Caitlin Fischer <caitlinfischer@google.com>
Commit-Queue: Nicolas Boichat <drinkcat@chromium.org>
Cr-Commit-Position: refs/heads/main@{#918351}