Set up a security.txt

[qwerty287]

See https://securitytxt.org/ for more info and an automatic generator (this does not have high priority as we have https://github.com/woodpecker-ci/.github/blob/main/SECURITY.md, but still would be nice to have I think).

Therefore, it would also be nice if a OpenPGP key could be created so security reporters can send their emails encrypted @woodpecker-ci/owner (maybe expiring on Dec 31 or Jan 1 so we can't have issues if owners change).