Masthead 1.6291: block editor fix

WordPress.org's theme reviewers sent back Colophon, the theme Masthead is built on, because the Site Editor showed "Block contains unexpected or invalid content." I checked every other theme I ship for the same problem. Masthead's block markup already matched what the editor saves, so the fix here is small.

• The blank page template had an HTML comment sitting outside any block. The editor reads text outside a block as an unrecognised block, which is what WordPress.org's review check reports as an incomplete template. I removed it.
• The main wrapper group set an "id" attribute the group block doesn't define. It now uses "anchor", the supported way to give a block an HTML id. The rendered id is the same.
• Nothing changes on the front end. Your site looks the same.

If you've customised any of these templates in the Site Editor, your edits are untouched. This update only replaces the files that ship with the theme.

Full changelog in readme.txt.