Use line comments instead of block comment for generated license headers
This makes it shorter. With the spdx license header in !225 (merged) the headers are generally 2 or 3 lines long, so we don't need to make it 50% longer by closing and opening the comment blog.
before with !225 (merged)
/* * SPDX-File.... * SPDX-Licen.... */
// SPDX-File... // SPDX-License...