[boost] [release] 1.71.0 closed for new libraries and breaking changes

The master branch is now closed for new libraries and breaking changes, and will be closing for major changes in a week and closed for the beta in two weeks. Also, if you're ready, you can also add your libraries' changes to the release notes at: https://github.com/boostorg/website/blob/master/feed/history/boost_1_71_0.qb... Thanks, -- The Boost Release Team