RFR(m): 8140281 deprecate Optional.get()

[Martin Buchholz]

No opinion on Optional.get() itself, but here's an opinion on the high
cost of deprecation.

If you deprecate the API today, and I have a library that is already
using the API, then I should add a @SuppressWarning("deprecation")
today and keep it there until the number of people using pre-jdk9 is
insignificant, when I can finally rename it.  How long is that?  These
days, I estimate at least 10 years.  But even 20 years is quite
possible.