openjfx21: use gradle_8 instead of gradle_7 to fix build (#458536)

  atLeast21 = lib.versionAtLeast featureVersion "21";

  atLeast23 = lib.versionAtLeast featureVersion "23";

  gradle_openjfx = if atLeast23 then gradle_8 else gradle_7;

  gradle_openjfx = if atLeast21 then gradle_8 else gradle_7;

in

assert lib.assertMsg (lib.pathExists sourceFile)