bug#51138: Increase the value of kill-ring-max

[Juri Linkov]

> The value of kill-ring-max was last increased in 1998, when it was
> doubled from 30 to 60.  See commit bffa4d92e152.
>
> Given that RAM is cheaper these days, how about doubling it to 120?
> That's the value I use locally, and it has saved my neck many times.

A long time ago I customized kill-ring-max to 1024,
and the only problem with such a big value is that
it produces too long submenu in "Paste from Kill Menu".
This is because menu-bar-update-yank-menu
truncates to the length of kill-ring-max,
but it should introduce a new variable
separately customized for the paste menu's size.