Consider using log/slog instead of logrus

[micahhausler]

Not a specific knock on logrus, but I only found 3 log lines using logrus in this library. Can we cut this third-party import and use standard library option?

$ grep logrus -r ./*
./go.mod:	github.com/sirupsen/logrus v1.9.3
./go.sum:github.com/sirupsen/logrus v1.9.3 h1:dueUQJ1C2q9oE3F7wvmSGAaVtTmUizReu6fjN8uqzbQ=
./go.sum:github.com/sirupsen/logrus v1.9.3/go.mod h1:naHLuLoDiP4jHNo9R0sCBMtWGeIprob74mVsIT4qYEQ=
./pkg/vex/vex.go:	"github.com/sirupsen/logrus"
./pkg/vex/vex.go:		logrus.Warn(err)
./pkg/vex/vex.go:	logrus.Warn("vex.StatementFromID is deprecated and will be removed in an upcoming version")
./pkg/vex/functions_files.go:	"github.com/sirupsen/logrus"
./pkg/vex/functions_files.go:		logrus.Info("Abriendo CSAF")

[puerco]

Sounds good to me, these are mostly vestigial from the original OpenVEX poc, we have tried to remove most of the output.