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/file/secondary.go

@@ -195,7 +195,7 @@ Restart:
 
 // jitter returns a random duration between [0,n) * time.Millisecond
 func jitter(n int) time.Duration {
-	r := rand.Intn(n)
+	r := rand.Intn(n) // #nosec G404 -- non-cryptographic jitter to spread transfer attempts.
 	return time.Duration(r) * time.Millisecond
 }