release a v5.15 based kernel as linux-librem5

a little todo list:

• DONE: wait until millipixels has migrated: https://software.pureos.net/search_pkg?term=millipixels and ideally call for testing one last time
• DONE: https://source.puri.sm/Librem5/linux-next/-/merge_requests/507 - create a release MR similar to https://source.puri.sm/Librem5/linux-next/-/commits/next/byzantium which includes in Breaks: megapixels (<< 0.17.0), and Fixes: #307 (closed) #289 (closed) #339 (closed) #342 (closed) #354 (closed)
• DONE: merge librem5-base!281 (merged) and librem5-base!280 (merged)
• DONE: librem5-base!284 (merged) create a release MR for librem5-base
• tag the kernel at the same time as librem5-base
• push next/byzantium to pureos/byzantium to free the space for v5.16 based kernels