bug#34789: mantemp.el should be obsoleted

[Paul Eggert]

Davis Herring writes, "One of your fixes was to progmodes/mantemp.el,
which (now that I know it exists) I'm pretty sure should be obsoleted;
as you may already know, C++ compilers haven't worked that way for any
standard library type (and most user-defined types) in a very long time."

I notice that no significant changes have been made to mantemp.el since
1997. As it's evidently useless nowadays, I propose that we move
mantemp.el to the obsolete area, and remove it in a later Emacs release.