chore(lint): enable gosec (#7792)

Enable "gosec" linter.

Exclude:

- All G115 (integer overflow) findings, to be fixed separately.

Add targeted gosec annotations for:

- non-crypto math/rand usage
- md5 used only for file change detection
- G114 ("net/http serve with no timeout settings"), to be fixed
  separately.

Other findings fixed.

Signed-off-by: Ville Vesilehto <ville@vesilehto.fi>

plugin/pkg/rand/rand.go

@@ -17,7 +17,7 @@ type Rand struct {
 
 // New returns a new Rand from seed.
 func New(seed int64) *Rand {
-	return &Rand{r: rand.New(rand.NewSource(seed))}
+	return &Rand{r: rand.New(rand.NewSource(seed))} // #nosec G404 -- non-cryptographic RNG by design (load balancing only).
 }
 
 // Int returns a non-negative pseudo-random int from the Source in Rand.r.