[boost] Release branch closing for new features

[ resending because it doesn't seem to have gone through ] So, we missed announcing this on Monday, so we're pushing back the deadline until Friday. On (tomorrow) Friday, Sep 29th, the release branch will close for new features, and be open only for bug fixes and doc fixes. On Tuesday, October 2nd, the release branch will close except by permission from the release managers. This is in preparation for the first 1.52.0 beta, scheduled for Monday, Oct 8th. -- The release managers