[$] Development statistics for the 6.15 kernel

The 6.14 kernel development cycle only brought in 11,003 non-merge
changesets, making it the slowest cycle since 4.0, which was released in
2015. The 6.15 kernel, instead, brought in 14,612 changesets, making it
the busiest release since 6.7, released at the beginning of 2024. The
kernel development process, in other words, is back up to full speed. The
6.15
release happened on May 25, so the time has come for the
obligatory look at where the changes in this release came from.