device-purism-librem5: upgrade to 1.15

This includes a new librem5-base config release, the most notable change being some mic echo cancellation changes in pulseaudio config.

Upstream changelog: https://source.puri.sm/Librem5/librem5-base/-/commit/351958ffc0ff21e31647d69e23e47aefdf2ec396

Note: I did not include the upstream change to disable wifi PM. I don't find the performance to be so bad that it's worth giving up some battery efficiency for.

Note: the librem5-base thing from purism has a new naming/versioning format, which is in line with some of their other projects, so the APKBUILD has been updated to support that too