The Warwick Foundation of Bucks County is making its final gifts. The county historical society and its Mercer Museum are getting the biggest one at $5 million.  The Heritage Conservancy is getting $1 million, and Doylestown Hospital and the Michener Museum are getting $500,000 each, as the foundation liquidates its assets and prepares to dissolve next year.  Earlier it pledged $15 million in money and a farm worth nearly $15 million to Delaware Valley College.