[boost] Master branch is now open for bug fixes

The master branch is now open for bug fixes and documentation changes. Other changes by release manager permission, as described in https://github.com/boostorg/wiki/wiki/Releases%3A-Beta-Merge-Policy <https://github.com/boostorg/wiki/wiki/Releases%3A-Beta-Merge-Policy> The next deadline: On April 5th, master closes for all changes. As always, the calendar is at https://www.boost.org/development/ <https://www.boost.org/development/> — Marshall