More than 20 stars—including newly announced Toby Keith, Alabama and ZZ Top’s Billy Gibbons—will come together at Nashville’s Bridgestone Arena on April 6 for a Merle Haggard tribute concert. Dubbed Sing Me Back Home: The Music of Merle Haggard, the concert will also feature performances from Willie Nelson, Kenny Chesney, Miranda Lambert, John Mellencamp, Dierks…