[release] Boost 1.78.0 Release Candidate 1

The first release candidates for the 1.78.0 release are now available at:
<https://boostorg.jfrog.io/artifactory/main/release/1.78.0/source/>

The SHA256 checksums are as follows:

94ced8b72956591c4775ae2207a9763d3600b30d9d7446562c552f0a14a63be7  boost_1_78_0_rc1.tar.gz
f22143b5528e081123c3c5ed437e92f648fe69748e95fa6e2bd41484e2986cc3  boost_1_78_0_rc1.zip
090cefea470bca990fa3f3ed793d865389426915b37a2a3258524a7258f0790c  boost_1_78_0_rc1.7z
8681f175d4bdb26c52222665793eef08490d7758529330f98d3b29dd0735bccc  boost_1_78_0_rc1.tar.bz2

As always, the release managers would appreciate it if you download the
candidate of your choice and give building it a try. Please report both
success and failure, and anything else that is noteworthy.

-- The Release managers