doc: releases: migration-guide-3.7: Rename BT_BREDR to BT_CLASSIC

Due to the source/header files are moved to subfolder "classic",
rename the kconfig option BT_BREDR to BT_CLASSIC to make the
folder name and kconfig option name consistent.

Signed-off-by: Lyle Zhu <lyle.zhu@nxp.com>