[boost] Master branch now closed for new libraries and breaking changes

Welcome to the start of the 1.88.0 release cycle. The release branch is now closed for new libraries and breaking changes to existing libraries. Still open for bug fixes and other routine changes to all libraries without release manager review. The next deadline is next Wednesday, when the release branch will be closed for major changes. As always, the calendar is here: https://www.boost.org/development/index.html — The release managers