[3.13] gh-115119: Defer removal of bundled libmpdec to 3.16 (GH-133997) (#136201)

Rename libmpdecimal -> libmpdec

see https://www.bytereef.org/mpdecimal/doc/libmpdec/index.html
(cherry picked from commit b19c9da401b9e81078103861f55e0762b93453f0)

Co-authored-by: Sergey B Kirpichev <skirpichev@gmail.com>