[boost] Release branch will be closing Monday night.

We will be “closing” the release branch late Monday night (Pacific Daylight time) in preparation for building a 1.56.0 release candidate on Tuesday morning. [ Actually, we’re just going to stop the automatic merging to the super-project, but … ] Please refrain from committing to the “master” branch of your libraries after 11:59 Monday night. (unless you have specific permission from the release managers) Thank you. — Marshall