feat!: new kernel config structure

BREAKING CHANGE: kernel definition in the configuration files has switched

from

  [[targets]]
  distro = { id = "Ubuntu", release = "18.04" }
  release_mask = ".*"

to

  [[targets]]
  distro = { id = "Ubuntu", release = "18.04" }
  kernel = { regex = ".*" }

examples/script/.out-of-tree.toml

@@ -7,4 +7,4 @@ script = "script.sh"
 
 [[targets]]
 distro = { id = "Ubuntu", release = "22.04" }
-release_mask = ".*"
+kernel = { regex = ".*" }