HTST max-age should be 2 years when preload is present

[yuryu]

HTST max-age is currently set to 3,600.

gcping/cmd/ping/main.go

Line 54 in 0f9908c

w.Header().Add("Strict-Transport-Security", "max-age=3600; includeSubdomains; preload")

However, according to https://hstspreload.org/, 2 years is recommended and 1 year is required as a minimum.