Mastodon performs at Concord Pavilion in Concord, Calif. on April 20, 2023. Sean Liming/STAFF. CONCORD, Calif. — Metal mainstays Mastodon and Gojira brought their Mega-Monsters Tour to the Concord Pavilion on Thursday, shaking the hills with driving beats and shredding guitars. Mastodon was appropriately ready for April 20 occasion with its…