west: spdx: allow to generate for different SPDX versions

When support for SPDX 2.3 was added, it effectively dropped support for
SPDX 2.2, which in retrospect was a bad idea since SPDX 2.2 is the
version that is the current ISO/IEC standard.
This commit adds a `--spdx-version` option to the `west spdx` command
so that users can generate SPDX 2.2 documents if they want.
Default is 2.3 given that's effectively what shipped for a few releases
now, including latest LTS.

Signed-off-by: Benjamin Cabé <benjamin@zephyrproject.org>