bootspec: drop ".conf" from BootEntry.filename

The boot loader systemd-boot removes ".conf" from file name of entry
configs, and determine which entry is the default entry.
However, bootspec, which is used by systemctl and bootctl did not
remove ".conf", then sometimes bootctl marks wrong entry as default.
This fixes the logic to choose the default entry in bootspec, to
match the logic used in systemd-boot boot loader.

Fixes #7727.