target.hardware: do not emit `src` attributes. These interfere with bitstream caching.

CI: fix cache key calculation. It turns out GitHub will not update the cache if the key already exists. Oops.

CI: remove all `if:` clauses. These appear to basically not work properly if merge queue is enabled.

CI: fix job skip conditions. Hopefully.

CI: only test software on push/pull_request if modified. This should speed up the tests a bit, and we still get the full suite run in the merge queue.

manual: install: phrasing.

manual: install: add instructions for FreeBSD.

software: drop support for Python 3.8. Even Debian oldstable has Python 3.9 now.

software: remove Python 3.7 compatibility shims. We dropped support for Python 3.7 some time ago, so these shims were completely unused.

support.plugin: fix the abominable workaround for Python 3.10+.